[GCC] Don't keep a StringRef to a temporary std::string
[GCC] Don't keep a StringRef to a temporary std::string

This fixes failures in asan builds and possibly other buildbots
as well, after SVN r330696.

Prior to that revision, the std::string was stored in another
variable, before assigning to a StringRef.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@330710 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang