Avoid overwriting the `swiftCore` target in the SourceKit build.
Instead, link to the explicit variant of the swiftCore. This tracks the
dependency better and enables multiple parallel cross-compilations of
the stdlib.
Implicitly link against swiftCore when linking against libdispatch.
Remove the extraneous link against the Blocks runtime on Linux. The
`add_sourcekit_executable` call already handles this. Ensure that we
enable the swift SDK overlay for libdispatch by sending it the path to
the swift compiler.
This is a NFC change that makes it easier to read SourceKit's main
CMakeLists.txt file since you only see "actions" rather than also this huge list
of helper routines.
