Fix use after free.

Found by asan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260814 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Lex/Pragma.cpp

@@ -1026,7 +1026,7 @@ public:
 
     diag::Flavor Flavor = WarningName[1] == 'W' ? diag::Flavor::WarningOrError
                                                 : diag::Flavor::Remark;
-    StringRef Group = WarningName.substr(2);
+    StringRef Group = StringRef(WarningName).substr(2);
     bool unknownDiag = false;
     if (Group == "everything") {
       // Special handling for pragma clang diagnostic ... "-Weverything".