Revert "Convert a few std::strings to StringRef."

This reverts commit r212342.

We can get a StringRef into the current Record, but not one in the bitcode
itself since the string is compressed in it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212356 91177308-0d34-0410-b5e6-96231b3b80d8