Report an error when -m<os>-version-min= does not specify a version.

Currently if you use -mmacosx-version-min or -mios-version-min without
specifying a version number, clang silently sets the minimum version to
"0.0.0". This is almost certainly not what was intended, so it is better
to report it as an error. rdar://problem/20433945

llvm-svn: 234270