swift234

Add _compiler_version build configuration

This configuration clause will suppress lex diagnostics and skip parsing
altogether if the code under the clause isn't active - the compiler must
have a repository version greater than or equal to the version given to
_compiler_version.

This option is only meant to be used sparingly and not to track the
Swift *language* version.

Example, if using a compiler versioned 700.0.28:

  #if _compiler_version("700.0.23")
    print("This code will compile for versions 700.0.23 and later.")
  #else
    This + code + will + not + be + parsed
  #endif

Included are new diagnostics for checking that the version is formatted
correctly and isn't empty.

New tests:
- Compiler version comparison unit tests
- Build configuration diagnostics
- Skipping parsing of code under inactive clauses

rdar://problem/22730282

Swift SVN r32195