PR15539: Record "evaluating if/elif condition" flag in the right place

The previous implementation missed the case where the elif condition was
evaluated from the context of an #ifdef that was false causing PR15539.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177345 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/PPExpressions.cpp b/lib/Lex/PPExpressions.cpp
index 49f4cbf..27ad7b4 100644
--- a/lib/Lex/PPExpressions.cpp
+++ b/lib/Lex/PPExpressions.cpp
@@ -24,6 +24,7 @@
 #include "clang/Lex/MacroInfo.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/SaveAndRestore.h"
 using namespace clang;
 
 namespace {
@@ -730,6 +731,7 @@
 /// to "!defined(X)" return X in IfNDefMacro.
 bool Preprocessor::
 EvaluateDirectiveExpression(IdentifierInfo *&IfNDefMacro) {
+  SaveAndRestore<bool> PPDir(ParsingIfOrElifDirective, true);
   // Save the current state of 'DisableMacroExpansion' and reset it to false. If
   // 'DisableMacroExpansion' is true, then we must be in a macro argument list
   // in which case a directive is undefined behavior.  We want macros to be able