When pretty printing attributes that have enumeration arguments, print the...

When pretty printing attributes that have enumeration arguments, print the enumerator identifier (as a string literal) instead of the internal enumerator integral value.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217771 91177308-0d34-0410-b5e6-96231b3b80d8