Storing std::strings in attributes simply doesn't work, we never call
the destructor. Use an array of StringRefs instead of std::strings and
copy the data into memory taken from the ASTContext.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260831 91177308-0d34-0410-b5e6-96231b3b80d8