swift-lang: shuffle the source directory structure (NFC)
This moves the swift-lang library out of the tools/SourceKit directory
and into the stdlib directory. As it stands, the library required that
the standard library and the SDK overlay was being built. This is
implicitly done when building the stdlib subdirectory. Furthermore, it
had an odd tie between the flags for Swift and SourceKit which now have
the logic separated. This clearly delineates the host and target
components in the repository.
