Fix UBSan issue with PPC::isValidCPUName
Fix UBSan issue with PPC::isValidCPUName

Apparently storing the pointer to a StringLiteral as
a StringRef caused this section of code to issue a ubsan
warning.  This will hopefully fix that.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@324687 91177308-0d34-0410-b5e6-96231b3b80d8

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