Adds a new type, CoreClrAnalyzerAssemblyLoader, that is responsible
for finding and loading analyzer assemblies on CoreCLR. This is used
in the CscCore and VbcCore CoreCLR-targeting projects.

Some of the references were outdated and some of the NuGet packages
were incorrectly authored. With the new references we don't need to
do any extra work to copy over the CoreCLR runtime -- it's copied as
a reference by NuGet 3 without any extra work.

Fixes #5267.

* This updates all references to nuget.exe to use the copy in the root.
* This deletes all NuGet.config and NuGet.exe files that are not in the root.
* This removes all references to NuGet.targets (which is obsolete and no longer exists)

Propagate Debugger… attributes from container of async/iterator method to the corresponding state machine type.

Fixes #4521.

These temps have special longlived kind "Using", but were produced as regular temps in the case involving dynamic.

Fixes #5323

This also disables IDE support for including .csx/.vbx files in a
C# or VB project, and fixes up IVsLanguageBlock service to handle
"incomplete" members (these frequently occur in C# when you parse
global statements in "regular" C# mode).

Fixes #4696.

Related to #1319.

Related to #2470.

Rogjt mpw we build a boostrap version of csc and vbc and bootstrap
with those running on Mono. This commit switches us to build a CoreCLR
compatible csc and then running the boostrap on CoreCLR

Right now the full set of Linux CoreCLR runtime assemblies is not
available on NuGet, so we substitute with some assemblies we stash in
our NuGet ZIP package.

Binder.BindExtensionMethod – do not reduce extension methods unless the method group is an argument of nameof().

Fixes #4970.

Add a new CompilationOption "ReportSuppressedDiagnostics" to enable reporting compiler diagnostics suppressed in source. Note that the command line compilers will report such suppressed diagnostics in the SARIF errorlog file but not on the console output.

This new compilation option was approved by the design team.

Fixes #5216