make sure to not warn about unused macros from -D

If a PCH is used for compilation, SourceManager::isInMainFile()
returns true even for the "<built-in>" predefines area. Using -D
only for the TU compilation may trigger -Wunused-macros for it.
It is admitedly a bit fishy to set a macro only for a TU and not
for the PCH, but this works fine if the PCH does not use the macro
(I couldn't find a statement on this for Clang, but GCC explicitly
allows this in the docs).

Differential Revision: https://reviews.llvm.org/D73846

clang/lib/Lex/PPDirectives.cpp

@@ -2802,7 +2802,9 @@ void Preprocessor::HandleDefineDirective(
   // 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()) &&
-      !MacroExpansionInDirectivesOverride) {
+      !MacroExpansionInDirectivesOverride &&
+      getSourceManager().getFileID(MI->getDefinitionLoc()) !=
+          getPredefinesFileID()) {
     MI->setIsWarnIfUnused(true);
     WarnUnusedMacroLocs.insert(MI->getDefinitionLoc());
   }

clang/test/PCH/cli-macro.c

@@ -0,0 +1,12 @@
+// Test this without pch.
+// RUN: %clang_cc1 -Wunused-macros -Dunused=1 -fsyntax-only -verify %s
+
+// Test with pch.
+// RUN: %clang_cc1 -Wunused-macros -emit-pch -o %t %s
+// RUN: %clang_cc1 -Wunused-macros -Dunused=1 -include-pch %t -fsyntax-only -verify %s
+
+// expected-no-diagnostics
+
+// -Dunused=1 is intentionally not set for the pch.
+// There still should be no unused warning for a macro from the command line.
+