Add new analyzer API (DiagnosticSuppressor) to allow programmatic suppression of analyzer and/or compiler non-error diagnostics

Fixes #20242 and #30172

Detailed design proposal [here](https://gist.github.com/mavasani/fcac17a9581b5c54cef8a689eeec954a).

Added public APIs with documentation comments:
```cs
namespace Microsoft.CodeAnalysis.Diagnostics
{
    /// <summary>
    /// The base type for diagnostic suppressors that can programmatically suppress analyzer and/or compiler non-error diagnostics.
    /// </summary>
    public abstract class DiagnosticSuppressor : DiagnosticAnalyzer
    {
        // Disallow suppressors from reporting diagnostics or registering analysis actions.
        public sealed override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray<DiagnosticDescriptor>.Empty;

        public sealed override void Initialize(AnalysisContext context) { }

        /// <summary>
        /// Returns a set of descriptors for the suppressions that this suppressor is capable of producing.
        /// </summary>
        public abstract ImmutableArray<SuppressionDescriptor> SupportedSuppressions { get; }

        /// <summary>
        /// Suppress analyzer and/or compiler non-error diagnostics reported for the compilation.
        /// This may be a subset of the full set of reported diagnostics, as an optimization for
        /// supporting incremental and partial analysis scenarios.
        /// A diagnostic is considered suppressible by a DiagnosticSuppressor if *all* of the following conditions are met:
        ///     1. Diagnostic is not already suppressed in source via pragma/suppress message attribute.
        ///     2. Diagnostic's <see cref="Diagnostic.DefaultSeverity"/> is not <see cref="DiagnosticSeverity.Error"/>.
        ///     3. Diagnostic is not tagged with <see cref="WellKnownDiagnosticTags.NotConfigurable"/> custom tag.
        /// </summary>
        public abstract void ReportSuppressions(SuppressionAnalysisContext context);
    }

    /// <summary>
    /// Provides a description about a programmatic suppression of a <see cref="Diagnostic"/> by a <see cref="DiagnosticSuppressor"/>.
    /// </summary>
    public sealed class SuppressionDescriptor : IEquatable<SuppressionDescriptor>
    {
        /// <summary>
        /// An unique identifier for the suppression.
        /// </summary>
        public string Id { get; }

        /// <summary>
        /// Identifier of the suppressed diagnostic, i.e. <see cref="Diagnostic.Id"/>.
        /// </summary>
        public string SuppressedDiagnosticId { get; }

        /// <summary>
        /// A localizable description about the suppression.
        /// </summary>
        public LocalizableString Description { get; }
    }

    /// <summary>
    /// Context for suppressing analyzer and/or compiler non-error diagnostics reported for the compilation.
    /// </summary>
    public struct SuppressionAnalysisContext
    {
        /// <summary>
        /// Suppressible analyzer and/or compiler non-error diagnostics reported for the compilation.
        /// This may be a subset of the full set of reported diagnostics, as an optimization for
        /// supporting incremental and partial analysis scenarios.
        /// A diagnostic is considered suppressible by a DiagnosticSuppressor if *all* of the following conditions are met:
        ///     1. Diagnostic is not already suppressed in source via pragma/suppress message attribute.
        ///     2. Diagnostic's <see cref="Diagnostic.DefaultSeverity"/> is not <see cref="DiagnosticSeverity.Error"/>.
        ///     3. Diagnostic is not tagged with <see cref="WellKnownDiagnosticTags.NotConfigurable"/> custom tag.
        /// </summary>
        public ImmutableArray<Diagnostic> ReportedDiagnostics { get; }

        /// <summary>
        /// Report a <see cref="Suppression"/> for a reported diagnostic.
        /// </summary>
        public void ReportSuppression(Suppression suppression);

        /// <summary>
        /// Gets a <see cref="SemanticModel"/> for the given <see cref="SyntaxTree"/>, which is shared across all analyzers.
        /// </summary>
        public SemanticModel GetSemanticModel(SyntaxTree syntaxTree);

        /// <summary>
        /// <see cref="CodeAnalysis.Compilation"/> for the context.
        /// </summary>
        public Compilation Compilation { get; }

        /// <summary>
        /// Options specified for the analysis.
        /// </summary>
        public AnalyzerOptions Options { get; }

        /// <summary>
        /// Token to check for requested cancellation of the analysis.
        /// </summary>
        public CancellationToken CancellationToken { get; }
    }

    /// <summary>
    /// Programmatic suppression of a <see cref="Diagnostic"/> by a <see cref="DiagnosticSuppressor"/>.
    /// </summary>
    public struct Suppression
    {
        /// <summary>
        /// Creates a suppression of a <see cref="Diagnostic"/> with the given <see cref="SuppressionDescriptor"/>.
        /// </summary>
        /// <param name="descriptor">
        /// Descriptor for the suppression, which must be from <see cref="DiagnosticSuppressor.SupportedSuppressions"/>
        /// for the <see cref="DiagnosticSuppressor"/> creating this suppression.
        /// </param>
        /// <param name="suppressedDiagnostic">
        /// <see cref="Diagnostic"/> to be suppressed, which must be from <see cref="SuppressionAnalysisContext.ReportedDiagnostics"/>
        /// for the suppression context in which this suppression is being created.</param>
        public static Suppression Create(SuppressionDescriptor descriptor, Diagnostic suppressedDiagnostic);

        /// <summary>
        /// Descriptor for this suppression.
        /// </summary>
        public SuppressionDescriptor Descriptor { get; }

        /// <summary>
        /// Diagnostic suppressed by this suppression.
        /// </summary>
        public Diagnostic SuppressedDiagnostic { get; }
    }
}
```

For batch compilation, suppressors always run after all the analyzer diagnostics have been computed.
For IDE partial/incremental analysis scenario, we may run the suppressors with partial diagnostics.
Suppressed diagnostics from diagnostic suppressors are equivalent to source suppressed diagnostics: they show up in the error list with "Suppression State" column as "Suppressed" and are also output to errorlog as suppressed diagnostics.