swift234

Normalize "darwin" target triples to "macosx" in LangOptions.

Clang does this in the driver. Why not Swift? Because (a) this is an
implementation detail that really only affects IRGen, and even then
somewhat incorrectly, and (b) Swift has a good handful of clients that
don't go through the driver to create an ASTContext (specifically, LLDB,
along with many of our testing tools).

rdar://problem/19779274

Swift SVN r25909