Add IOperation APIs for nameof and throw expressions

Add IDefaultCaseClause for default case clause

Fixes https://github.com/dotnet/roslyn/issues/19954 and https://github.com/dotnet/roslyn/issues/20125

Fixes https://github.com/dotnet/roslyn/issues/20312

Merge master to features/ioperation

Remove allocation of char array

Provide more information for nested types in NavigateTo to help disambiguate them

Adding some orphaned tests back.

Merge dev15.3.x to master

Allow people to create lists at the common layer, instead of having to go through the C# and VB layers.

Move namespace name checking out of the parser.

Remove closed assets

Updated WorkItemAttribute documentation

This removes a couple of files which only existed to support the roslyn-internal
submodule relationship. That is no longer maintained hence these are not
necessary anymore.

Added explicit conversion expression tests for C#.

Vb implicit conversions

The construction arguments were misleading with respect to our practices. I
updated the names and documentation so new contributors can follow along more
easily.

Merge master to features/ioperation

* dotnet/features/ioperation: (60 commits)
  Fix unit test failures from merge conflict resolution
  removed double empty line
  remove duplicated test
  Fix build break in VB
  Address some more review feedback and fix one more break.
  Fix build break from merge conflict resolution
  added vb tests
  fixed failed tests
  added csharp unary operator expression test
  missed adding simple to one comment
  addressed PR feedback
  Add entries for dev15.3.x branch in readme.md
  Don't assume a document has SyntaxFactsService
  Skip CSharpInteractive.TypingHelpDirectiveWorks test
  Don't fetch every PR ref for every PR job
  create dump files when VS instance crashes or hangs (integration tests) (#20129)
  Clear completion pattern matcher caches on solution close (#20463)
  made all tests pass
  Address PR feedback from Chuck
  Cast the correct object to IVsBrowseObjectContext
  ...