ASTGen always builds with the host Swift compiler, without requiring
bootstrapping, and is enabled in more places. Move the regex literal
parsing logic there so it is enabled in more host environments, and
makes use of CMake's Swift support. Enable all of the regex literal
tests when ASTGen is built, to ensure everything is working.
Remove the "AST" and "Parse" Swift modules from SwiftCompilerSources,
because they are no longer needed.
Fix a crash that could occur when performing
completion at the start of an accessor body.
Previously we assumed `CodeCompletion` would never
be null due to function body skipping in the first
pass of code completion. However with the
introduction of the ability to avoid skipping in
certain cases, it might be now be null if we need
to avoid skipping. Found by the stress tester.
rdar://95772803
