do not emit -Wunused-macros warnings in -frewrite-includes mode (PR15614)

-frewrite-includes calls PP.SetMacroExpansionOnlyInDirectives() to avoid
macro expansions that are useless in that mode, but this can lead
to -Wunused-macros false positives. As -frewrite-includes does not emit
normal warnings, block -Wunused-macros too.

Differential Revision: https://reviews.llvm.org/D65371

llvm-svn: 372026

clang/lib/Lex/PPDirectives.cpp

@@ -2782,7 +2782,8 @@ void Preprocessor::HandleDefineDirective(
   // If we need warning for not using the macro, add its location in the
   // warn-because-unused-macro set. If it gets used it will be removed from set.
   if (getSourceManager().isInMainFile(MI->getDefinitionLoc()) &&
-      !Diags->isIgnored(diag::pp_macro_not_used, MI->getDefinitionLoc())) {
+      !Diags->isIgnored(diag::pp_macro_not_used, MI->getDefinitionLoc()) &&
+      !MacroExpansionInDirectivesOverride) {
     MI->setIsWarnIfUnused(true);
     WarnUnusedMacroLocs.insert(MI->getDefinitionLoc());
   }

clang/test/Frontend/rewrite-includes-warnings.c

@@ -1,4 +1,7 @@
-// RUN: %clang_cc1 -verify -Wall -Wextra -E -frewrite-includes %s
+// RUN: %clang_cc1 -verify -Wall -Wextra -Wunused-macros -E -frewrite-includes %s
 // expected-no-diagnostics
 
 #pragma GCC visibility push (default)
+
+#define USED_MACRO 1
+int test() { return USED_MACRO; }