[leaks] Fix a leak recently introduced to the pragma handling. This

whole code would be better with std::unique_ptr managing the lifetimes
of the handlers, but I wanted to make a targeted fix to the leaks first.
With this change, all of the Clang preprocessor tests are leak free with
LSan.

llvm-svn: 207872

clang/lib/Lex/Pragma.cpp

@@ -1403,6 +1403,7 @@ void Preprocessor::IgnorePragmas() {
            "Invalid namespace, registered as a regular pragma handler!");
     if (PragmaHandler *Existing = STDCNamespace->FindHandler("", false)) {
       RemovePragmaHandler("STDC", Existing);
+      delete Existing;
     }
   }
   AddPragmaHandler("STDC", new EmptyPragmaHandler());