Fixes #41457.

Filed #41539 to track the .NET 5 annotation changes

This changes all of our uses of `NETCOREAPP3_1` to `NETCOREAPP`. There
is no value in distinguishing the exact .NET Core version we use in our
source tree as we only target one version. Also going forward it's not
expected that our .NET Core specific code will be version specific. If
that arises we can go back to version specific defines for those cases.

This also removes the last references to `netcoreapp1.1`,
`netcoreapp2.1` and `netcoreapp3.0` from our code.

Fixes #41402

When performing analyzer execution from IDE for open files using [CompilationWithAnalyzers.GetAnalyzerSemanticDiagnosticsAsync](http://sourceroslyn.io/#q=CompilationWithAnalyzers.GetAnalyzerSemanticDiagnosticsAsync) API, we were not properly reseting the analyzer state after executing operation actions. This caused any subsequent operation block registrations by the same analyzer to be skipped in the analysis callbacks. This issue does not affect command line analyzer execution as we always run the analyzers on the entire compilation, and hence do not perform any partial state tracking.
Verified that the added unit test fails prior to the fix.

* MethodSymbol and FieldSymbol equality fixes:
 - Defer the equality of method/field symbol to the substituted symbol when compared
 - Make the hashcode of substituted method/field defer to the original definition

Rename from IncludeNonNullableReferenceTypeModifier to IncludeNotNullableReferenceTypeModifier (#41332)

This changes the C# compiler so that it multi-targets between
`netstandard2.0` and `netcoreapp3.1`.

Closes #41304

This API has always accepted null as a shortcut for an empty array, so
this annotates it accordingly.

Had to clean up a few nullable annotations now that we are compiling
agaist `netcoreapp3.1` and hence get the full value of the framework
annotations.

This is also problematic though because there are now two places where
the compiler can see nullable attributes that are directly used by the
developer. For example `NotNullWhenAttribute`. This is both defined in
our assemblies for non-netcoreapp target frameworks and provided by the
SDK when targeting `netcoreapp3.1`.

This causes a problem for assemblies which have the following
characteristics:

1. Target `netcoreapp3.1`
1. Reference an assembly targeting `netstandard2.0` which uses our
nullable attributes definition
1. Has IVT into (2) above

These properties essentially define all of our unit test assemblies. In
that environment it's not possible to use nullable attributes in code
because the compiler can't disambiguate which definition of
`NotNullWhenAttribute` to use. This meant I had to temporarily remove a
few attributes until we can complete #40766.