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