Use llvm::raw_string_ostream instead of std::stringstream (NFC)
Use llvm::raw_string_ostream instead of std::stringstream (NFC)

As a side effect, this avoid having to call .data() on the StringRef.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@283416 91177308-0d34-0410-b5e6-96231b3b80d8

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