[Clang][Preprocessor] Enable macro expansion **only** in directives when parsing preprocessed sources

The preprocessor does not expand macro-identifiers in #pragma
directives.
When we preprocess & parse the code, clang expands the macros in
the directives.
When we parse already preprocessed code, clang still has to expand the
macros in the directives.
This also means that we're not always able to parse the preprocessor's
output without preserving the definitions (-dD).

Author: jmmartinez

clang/include/clang/Frontend/FrontendAction.h

@@ -85,7 +85,7 @@ class FrontendAction {
   /// EndSourceFileAction() will not be called.
   virtual bool BeginSourceFileAction(CompilerInstance &CI) {
     if (CurrentInput.isPreprocessed())
-      CI.getPreprocessor().SetEnableMacroExpansion(false);
+      CI.getPreprocessor().SetMacroExpansionOnlyInDirectives();
     return true;
   }
 
@@ -103,7 +103,7 @@ class FrontendAction {
   virtual void EndSourceFileAction() {
     if (CurrentInput.isPreprocessed())
       // reset the preprocessor macro expansion to the default
-      getCompilerInstance().getPreprocessor().SetEnableMacroExpansion(true);
+      getCompilerInstance().getPreprocessor().SetEnableMacroExpansion();
   }
 
   /// Callback at the end of processing a single input, to determine

clang/include/clang/Lex/Preprocessor.h

@@ -1837,7 +1837,9 @@ class Preprocessor {
     MacroExpansionInDirectivesOverride = true;
   }
 
-  void SetEnableMacroExpansion(bool Enable) { DisableMacroExpansion = !Enable; }
+  void SetEnableMacroExpansion() {
+    DisableMacroExpansion = MacroExpansionInDirectivesOverride = false;
+  }
 
   /// Peeks ahead N tokens and returns that token without consuming any
   /// tokens.

clang/test/Preprocessor/preprocess-pragma-cpp-output.c

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -E -x c %s | FileCheck %s
 // RUN: %clang_cc1 -x c -fsyntax-only %s -verify
+// RUN: %clang_cc1 -x cpp-output -fsyntax-only -verify %s
 // expected-no-diagnostics
-// RUN: %clang_cc1 -x cpp-output -fsyntax-only -verify=fail %s
 
 // The preprocessor does not expand macro-identifiers in #pragma directives.
 // When we preprocess & parse the code, clang expands the macros in directives.
@@ -14,7 +14,7 @@
 
 void foo() {
     // CHECK: #pragma unroll FACTOR
-    #pragma unroll FACTOR // fail-error{{use of undeclared identifier}}
+    #pragma unroll FACTOR
     for(;;) {
     }
     return;