Ignore macro expansions when scanning for fallthrough comments

[tbaeder]

Summary

When scanning for fallthrough comments, it's not useful to go to the definition of a macro. We instead only want to search at the expanded source code location. Includes a test that failed before.

[tbaeder]

fallthrough comment detection has been reverted, so abandon this in the meantime.

clang/lib/Sema/AnalysisBasedWarnings.cpp

static const Stmt *getLastStmt(const CFGBlock &B) {
           return SW->getSubStmt();
 
       return nullptr;
     }
 
     bool isFollowedByFallThroughComment(const Stmt *Statement) {
       // Try to detect whether the fallthough is marked by a comment like
       // /*FALLTHOUGH*/.
+      auto Loc = S.getSourceManager().getExpansionLoc(Statement->getEndLoc());
       bool Invalid;
-      const char *SourceData = S.getSourceManager().getCharacterData(
-          Statement->getEndLoc(), &Invalid);
+      const char *SourceData =
+          S.getSourceManager().getCharacterData(Loc, &Invalid);
       if (Invalid)
         return false;
       const char *LineStart = SourceData;
       for (;;) {
         LineStart = strchr(LineStart, '\n');
         if (LineStart == nullptr)
           return false;
         ++LineStart; // Start of next line.

clang/test/Sema/fallthrough-comment.c

 // RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wimplicit-fallthrough %s
 
+#define SOME_CONSTANT 1
+
 int fallthrough_comment(int n) {
   switch (n) {
   case 0:
     n++;
     // FALLTHROUGH
   case 1:
     n++;
 
     /*fall-through.*/
 
-  case 2:
+  case 3:
+    n += SOME_CONSTANT;
+    /* FALL THRU */
+
+  case 4:
     n++;
-  case 3: // expected-warning{{unannotated fall-through between switch labels}} expected-note{{insert '__attribute__((fallthrough));' to silence this warning}} expected-note{{insert 'break;' to avoid fall-through}}
+  case 5: // expected-warning{{unannotated fall-through between switch labels}} expected-note{{insert '__attribute__((fallthrough));' to silence this warning}} expected-note{{insert 'break;' to avoid fall-through}}
     n++;
     break;
   }
   return n;
 }