swift234

Review changes for _compiler_version

A couple of small tweaks to _compiler_version based on review comments:
- Fix &&/|| rejection to work with _compiler_version on either side of the
expression. Also add some test cases around this.
- Use clang/LLVM facilities for isdigit and atoi.
- Assert if parsing an invalid version string and there is no diagnostic
engine.
- Clean up some crumbs in the CMake configs.

rdar://problem/22730282

Swift SVN r32212