swift234

[SourceKit] Register optional sources in CMake

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.