* Fix nullable annotations for ValueSource

* Refactor WeakValueSource

* Replace OptionalValueSource<T> with ValueSource<Optional<T>>

Some reference assemblies omit information about private fields. To
avoid treating structures in these assemblies as immutable, we only
consider types immutable if they contain information about one or more
private fields.

This is a workaround for issues like dotnet/standard#678.

Update FindImplementationsForInterfaceMemberAsync to correctly handle ambiguous type returns (#35853)

FindImplementationsForInterfaceMemberAsync checks for forwarded types using SymbolFinder.OriginalSymbolsMatch, which expects non-null compilation for both the project containing the interface and the project containing the symbol to compare to. Until now, most instances have found equivalence without falling through, but in cases where the type forwarding isn't completely verified a non-null compilation for the interface symbol is needed as well. See VerifyForwardedTypes for the logic being used.

In #35786 the type System.Text.Encoding is provided by a a NuGet reference and could be ambiguous based on the build target. The OOP service attempts to find the correct type resolution but needs the original symbol compilation to verify.

Without this fix, FindAllReferences may cause a null deref and fail.

Fixes #35786

(DisposableFieldsShouldBeDisposed)

We will now flag disposable creations in methods that do not escape the
method body (return value, out/ref params, assign to field/property, add
to collection, etc.). Additionally, we also flag disposable fields of
disposable types that are not disposed in the type's Dispose method.
Ported analyzers share the core dataflow analysis computation with the
analyzers in the roslyn-analyzers repo
(Microsoft.CodeAnalysis.FlowAnalysis.Utilities). This PR does not add
any code fixes. We already have a code
refactoring that offers wrapping a disposable creation within a using
statement. I am hoping to extend that to handle more cases and also
possibly convert it to a code fix in follow-up PR(s).

(DisposableFieldsShouldBeDisposed)

We will now flag disposable creations in methods that do not escape the
method body (return value, out/ref params, assign to field/property, add
to collection, etc.). Additionally, we also flag disposable fields of
disposable types that are not disposed in the type's Dispose method.
Ported analyzers share the core dataflow analysis computation with the
analyzers in the roslyn-analyzers repo
(Microsoft.CodeAnalysis.FlowAnalysis.Utilities). This PR does not add
any code fixes. We already have a code
refactoring that offers wrapping a disposable creation within a using
statement. I am hoping to extend that to handle more cases and also
possibly convert it to a code fix in follow-up PR(s).

- Do not generate implementation for sealed or static members.
- Do not generate implicit implementation for non-public members.
- Do not generate explicit implementation for inaccessible members.

- Include non-abstract definition into the implementations result.
- Include implementations from derived interfaces into the result.

use SymbolFinder.OriginalSymbolsMatch.

SymbolFinder.OriginalSymbolsMatch has heuristics to determine if types are
the same even if coming from a different assembly. This helps in cases
where GoToImplementation needs to resolve across projects that get
types like System.Threading.Tasks.Task from different assemblies, but
are equivalent and fulfill requirements for interface implementation.

This method is only recursive in one limited scenario, which is now
specially handled to avoid the need to track a visited set.

Fixes #28511

* Rename Foo to Goo

* Rename ifoo to igoo

* Add back BOMs

Improve the implementation of GetTypesImmediatelyDerivedFromClassesAsync by creating far less source symbols when searching for matches.