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.
Because we don't form a type-checked call to the
Regex initializer in the AST, we need to explicitly
handle the availability checking for `Regex<Output>`
and the initializer we're implicitly calling.
rdar://92156542
