swift234

[stdlib] Link an essential core library subset

To support layering improvements, add an (optionally built) "essential
core" library containing the minimum set of files required just to link
successfully.  Right now all those object files are linked again into
swift_stdlib_core; it would probably be better to link the essential
core into the core, but my CMake/Swift Driver fu is not strong enough
and this will at least help during development.

Along the way, removed the obsolete SwiftStdlibSources.txt file, as we
only have to support one build system now.

Swift SVN r14792