Support ExperimentalAttribute

PDB fixes and testing

Always show snippets should not exclude non-snippet items

Fix RPS regression due to overly aggressive assertions.

Merge dev15.2.x to master

Fix crash in lightbulbs in projection scenarios.

A previous change of mine moved it so that we try to grab the user's selectoin as the first
thing we do when queried in lightbulbs.  This was to make it so we would not call back into
the UI thread from a BG thread.  Unfortunately, the code i moved was not safe to call if the
query was a projection buffer.  This wasn't a problem previously because there was a prior
check that ran that would cause us to bail out.

I've now reordered things so that the check comes first, and then we get the selection afterwards.

Read rule set path from options

Don't offer 'convert if to switch' for basic if-tests.

Only offer the 'add named parameter' refactoring when on the start of the argument being passed in.

#18883

Add solution dependencies for portable tests to TestUtilities.Desktop

Cherry-picking #17544 to the dev15.2 branch.

Conflicts:
	src/Compilers/CSharp/Test/Semantic/Semantics/OutVarTests.cs

Fix code break caused by misordered merges.

Fixes #18653

Make all of FindUsagesContext async.

Fix CodeModel crash if the file is closed

Conflicts:
	src/Compilers/CSharp/Test/Semantic/Semantics/OutVarTests.cs

Unresolvable conflict not indicated when renaming identifier.

Fix failures to dispose PinnedRemotableDataScope

Move all expensive FAR work into the FAR engine, and remove from teh FAR window.