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.

llvm-svn: 324687
diff --git a/clang/lib/Basic/Targets/PPC.cpp b/clang/lib/Basic/Targets/PPC.cpp
index 40211db..3516a73 100644
--- a/clang/lib/Basic/Targets/PPC.cpp
+++ b/clang/lib/Basic/Targets/PPC.cpp
@@ -493,8 +493,7 @@
 };
 
 bool PPCTargetInfo::isValidCPUName(StringRef Name) const {
-  const StringRef *FoundName = llvm::find(ValidCPUNames, Name);
-  return FoundName != std::end(ValidCPUNames);
+  return llvm::find(ValidCPUNames, Name) != std::end(ValidCPUNames);
 }
 
 void PPCTargetInfo::fillValidCPUList(SmallVectorImpl<StringRef> &Values) const {