Fix more fallout from the introduction of "macosx" and "ios" triples.

The Darwin toolchain constructor was assuming that all Darwin triples would
have an OS string starting with "darwin".  Triples starting with "macosx"
would misinterpret the version number, and "ios" triples would completely
miss the version number (or worse) because the OS name is not 6 characters
long.  We lose some sanity checking of triple strings here, since the
Triple.getOSVersion function doesn't do all the checking that the previous
code did, but this still seems like a step in the right direction.

llvm-svn: 149422