When generating Spelling enumeration values, do not generate identifiers in...

When generating Spelling enumeration values, do not generate identifiers in the reserved namespace. Strip underscores as appropriate, taking care to not create duplicate identifiers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199414 91177308-0d34-0410-b5e6-96231b3b80d8