[flang][driver] Report prescanning diags during syntax-only parsing

Ensure diagnostics from the prescanner are reported when running `flang-new -fsyntax-only` (i.e. only syntax parsing).
This keeps the diagnostics output of flang-new consistent with `f18 -fparse-only` when running the syntax parsing action, ParseSyntaxOnlyAction.

Summary of changes:
- Modify ParseSyntaxOnlyAction::ExecuteAction to report diagnostics

Differential Revision: https://reviews.llvm.org/D95220

Author: Faris Rehman

flang/lib/Frontend/FrontendActions.cpp

@@ -94,6 +94,9 @@ void ParseSyntaxOnlyAction::ExecuteAction() {
     return;
   }
 
+  // Report the diagnostics from parsing
+  ci.parsing().messages().Emit(llvm::errs(), ci.allCookedSources());
+
   auto &parseTree{*ci.parsing().parseTree()};
 
   // Prepare semantics

flang/test/Frontend/preprocessor-diag.f90

@@ -0,0 +1,17 @@
+! Test that the driver correctly reports diagnostics from the prescanner. The contents of the include file are irrelevant here.
+
+! Test with -E (i.e. PrintPreprocessedAction, stops after prescanning)
+! 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 with -fsyntax-only (i.e. ParseSyntaxOnlyAction, stops after semantic checks)
+! RUN: %f18 -fparse-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang-new -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
+! RUN: %flang-new -fc1 -fsyntax-only -I %S/Inputs/ %s 2>&1 | FileCheck %s
+
+! CHECK: prescanner-diag.f90:14:20: #include: extra stuff ignored after file name
+#include <empty.h> comment
+! CHECK: prescanner-diag.f90:16:20: #include: extra stuff ignored after file name
+#include "empty.h" comment
+end