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.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
