Patch to wrap up '_' as separator in version numbers

Patch to wrap up '_' as separator in version numbers
in availability attribute by preserving this info.
in VersionTuple and using it in pretty printing of attributes
and yet using '.' as separator when diagnosing unavailable
message calls. rdar://18490958

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219124 91177308-0d34-0410-b5e6-96231b3b80d8