Cherry-pick 7496ec0a: Refactoring/Simplification: Walk language specific syntax trees directly instead by visitor pattern.

Cherry picked 5d21c614 Filter  assemblies in completion list that are already "InternalsVisibleTo" by looking up of InternalsVisibleTo attribute instances of the current project.

More build / SDK cleanup

Change for `fixed` statement to emit more optimal code.

Merge dev15.3.x to master

Fix IntelliSense in VSSDK projects that do not produce a VSIX

Workaround for #20543

fix protected event handler

Improve multiple SymbolFinder calls by 2x.

This wasn't truly used anymore. Removing the complication from our
build.

Couple of changes

- Advertise the UnitTestContainer capability
- Remove the now unnecessary legacy F5 settings for xunit procesess

Post SDK cleanup

Cleanup the test-determinism script

Work around uninstall issue with our VSIX

There is an issue uninstalling our VSIX via the standard tooling at the
moment. Using the wildfire approach for now to unblock PRs

Launch settings

Improving IntegrationTest infrastructure

The determinism test script was authored when our build still dumped everything into
a common directory. That meant the process of finding the DLLs / EXEs to check for
deterministic output was rather complex because we had to sort out the DLLS / EXES
we weren't building.

Now that our build is more structured it's simple to find the content we are directly
building: just search the Obj directory.

Remove stale branches from Readme.md

Use an ordinal comparison in GetSourceSymbolsChecksumAsync