TableGen: Generate valid identifiers for anonymous records

Backends like OptParserEmitter assume that record names can be used as valid
identifiers.

The period '.' in generated anonymous names broke that assumption, causing a
build-time error and in practice forcing all records to be named.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197869 91177308-0d34-0410-b5e6-96231b3b80d8