Fix isInSystemMacro in presence of macro and pasted token

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Summary

When a warning is raised from the expansion of a system macro that involves pasted token, there was still situations were they were not skipped, as showcased by this issue: https://bugzilla.redhat.com/show_bug.cgi?id=1472437

[serge-sans-paille]

@rsmith up!

[serge-sans-paille]

@rsmith : up :-)

[serge-sans-paille]

@rsmith : up :-)

[serge-sans-paille]

Updated with clang-formated diff

clang/include/clang/Basic/SourceManager.h

public:
 
   /// Returns whether \p Loc is expanded from a macro in a system header.
   bool isInSystemMacro(SourceLocation loc) const {
     if (!loc.isMacroID())
       return false;
 
     // This happens when the macro is the result of a paste, in that case
     // its spelling is the scratch memory, so we take the parent context.
-    if (isWrittenInScratchSpace(getSpellingLoc(loc)))
-      return isInSystemHeader(getSpellingLoc(getImmediateMacroCallerLoc(loc)));
+    // There can be several level of token pasting.
+    if (isWrittenInScratchSpace(getSpellingLoc(loc))) {
+      do {
+        loc = getImmediateMacroCallerLoc(loc);
+      } while (isWrittenInScratchSpace(getSpellingLoc(loc)));
+      return isInSystemMacro(loc);
+    }
 
     return isInSystemHeader(getSpellingLoc(loc));
   }
 
   /// The size of the SLocEntry that \p FID represents.
   unsigned getFileIDSize(FileID FID) const;
 
   /// Given a specific FileID, returns true if \p Loc is inside that

clang/test/Misc/no-warn-in-system-macro.c

 // RUN: %clang_cc1 -isystem %S -Wdouble-promotion -fsyntax-only %s  2>&1 | FileCheck -allow-empty %s
 // CHECK-NOT: warning:
 
 #include <no-warn-in-system-macro.c.inc>
 
+#define MACRO(x) x
+
 int main(void)
 {
   double foo = 1.0;
 
   if (isnan(foo))
     return 1;
-  return 0;
+
+        MACRO(isnan(foo));
+
+        return 0;
 }