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.
Ban space and tab as the last character of a
`/.../` regex literal, unless escaped with a
backslash. This matches the banning of space and
tab as the first character, and helps avoid breaking
source in even more cases.
This is unfortunately needed to ensure we correctly
re-lex regex literal tokens correctly, which is
needed for diagnostic logic to correctly compute
source ranges.
rdar://92469692