Changed the code comment mentioning "null literal" and "special case" and replaced it with a more general description.

Enable debug information by default in CSI

Fix bug when caret is at the end of an argument

Fix perf regression in gotodef.

Move DocumentHighlights data types down to the 'Features' layer.

Fix FQN for CodeLens (for test discovery)

Reduce overhead from memory mapped files

Test base

Check for missing hierarchies when mapping to Roslyn projects

Add a common base type of IDE and diagnostics VS performance tests. 

Fixes #19493

Convert if-to-switch.