[modules] Follow the C++ standard's rule for linkage of enumerators: they have

[modules] Follow the C++ standard's rule for linkage of enumerators: they have
the linkage of the enumeration. For enumerators of unnamed enumerations, extend
the -Wmodules-ambiguous-internal-linkage extension to allow selecting an
arbitrary enumerator (but only if they all have the same value, otherwise it's
ambiguous).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@253010 91177308-0d34-0410-b5e6-96231b3b80d8