[flang][driver] Make the driver report diagnostics from the prescanner

This patch makes sure that diagnostics from the prescanner are reported
when running `flang-new -E` (i.e. only the preprocessor phase is
requested). More specifically, the `PrintPreprocessedAction` action is
updated.

With this patch we make sure that the `f18` and `flang-new` provide
identical output when running the preprocessor and the prescanner
generates diagnostics.

Differential Revision: https://reviews.llvm.org/D94782

Author: banach-space

flang/lib/Frontend/FrontendActions.cpp

@@ -61,6 +61,9 @@ void PrintPreprocessedAction::ExecuteAction() {
     return;
   }
 
+  // Print diagnostics from the preprocessor
+  ci.parsing().messages().Emit(llvm::errs(), ci.allCookedSources());
+
   // Create a file and save the preprocessed output there
   if (auto os{ci.CreateDefaultOutputFile(
           /*Binary=*/true, /*InFile=*/GetCurrentFileOrBufferName())}) {

flang/test/Frontend/Inputs/empty.h

@@ -0,0 +1,11 @@
+! RUN: %f18 -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang-new -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang-new -fc1 -E -I %S/Inputs/ %s 2>&1 | FileCheck %s
+
+! Test that the driver correctly reports diagnostics from the prescanner. The contents of the include file are irrelevant here.
+
+! CHECK: preprocessor-diag.f90:8:20: #include: extra stuff ignored after file name
+#include <empty.h> comment
+! CHECK: preprocessor-diag.f90:10:20: #include: extra stuff ignored after file name
+#include "empty.h" comment
+end