LLVM's CMake modules include a function `llvm_process_sources()`,
which (among other things) verifies that all source files in a
directory are either included in the list of source files to process,
or are included in a list `LLVM_OPTIONAL_SOURCES`.
SourceKit's CMake functions make use of this LLVM function, but do
not register any files as "optional". When attempting to configure
CMake to include SourceKit on a Linux host machine, source files
that are only included on Darwin host machines cause this function
to raise an error.
Mark Darwin-only SourceKit files as "optional" to avoid the error.
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.
