Avoid storing Twines.

While there nested ifs into a helper function. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205108 91177308-0d34-0410-b5e6-96231b3b80d8
TableGen: don't save a StringRef to a local std::string.

This caused a failure in some Windows builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205109 91177308-0d34-0410-b5e6-96231b3b80d8
tblgen: Twinify PrintFatalError.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205110 91177308-0d34-0410-b5e6-96231b3b80d8