Replace usage of std::stringstream with raw_string_ostream

Typically we don't use the stringstream, so instead use
raw_string_stream.  Additionally, the dependent function
changed to use raw_ostream.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315950 91177308-0d34-0410-b5e6-96231b3b80d8