Address PR comments, add a unit test

Most of the changes are NFCs that address the PR comments. I've also
added a unit test in flang/unittests/Frontend/FrontendActionTest.cpp.
For that I had to extend the `CompilerInstance` class.

Author: banach-space

clang/lib/Driver/Driver.cpp

@@ -43,7 +43,6 @@
 #include "ToolChains/PS4CPU.h"
 #include "ToolChains/RISCVToolchain.h"
 #include "ToolChains/Solaris.h"
-
 #include "ToolChains/TCE.h"
 #include "ToolChains/VEToolchain.h"
 #include "ToolChains/WebAssembly.h"

flang/include/flang/Frontend/CompilerInstance.h

@@ -250,6 +250,13 @@ class CompilerInstance {
   void WriteOutputStream(const std::string &message) {
     *outputStream_ << message;
   }
+
+  /// Get the user specified output stream.
+  llvm::raw_pwrite_stream &GetOutputStream() {
+    assert(outputStream_ &&
+        "Compiler instance has no user-specified output stream!");
+    return *outputStream_;
+  }
 };
 
 } // end namespace Fortran::frontend

flang/lib/Frontend/FrontendActions.cpp

@@ -367,7 +367,6 @@ void GetSymbolsSourcesAction::ExecuteAction() {
 
 // Translate front-end KINDs for use in the IR and code gen. Extracted from
 // bbc.cpp.
-// TODO: How to share this with bbc.cpp?
 static std::vector<fir::KindTy>
 fromDefaultKinds(const Fortran::common::IntrinsicTypeDefaultKinds &defKinds) {
   return {static_cast<fir::KindTy>(defKinds.GetDefaultKind(
@@ -400,7 +399,7 @@ void EmitFirAction::ExecuteAction() {
       ci.invocation().semanticsContext().intrinsics(), ci.parsing().allCooked(),
       "", kindMap);
 
-  // Create PFT and lower it to FIR
+  // Create a parse tree and lower it to FIR
   auto &parseTree{*ci.parsing().parseTree()};
   lb.lower(parseTree, ci.invocation().semanticsContext());
 
@@ -410,19 +409,32 @@ void EmitFirAction::ExecuteAction() {
   pm.addPass(std::make_unique<Fortran::lower::VerifierPass>());
   mlir::ModuleOp mlirModule = lb.getModule();
   if (mlir::failed(pm.run(mlirModule))) {
-    // TODO: Replace with a diagnostic
-    llvm::errs() << "FATAL: verification of lowering to FIR failed";
+    unsigned diagID =
+        ci.diagnostics().getCustomDiagID(clang::DiagnosticsEngine::Error,
+            "verification of lowering to FIR failed");
+    ci.diagnostics().Report(diagID);
+    return;
   }
 
-  // Print the output
+  // Print the output. If a pre-defined output stream exists, dump the MLIR
+  // content there.
+  if (!ci.IsOutputStreamNull()) {
+    mlirModule.print(ci.GetOutputStream());
+    return;
+  }
+
+  // ... otherwise, print to a file.
   auto os{ci.CreateDefaultOutputFile(
-          /*Binary=*/true, /*InFile=*/GetCurrentFileOrBufferName(), "mlir")};
+      /*Binary=*/true, /*InFile=*/GetCurrentFileOrBufferName(), "mlir")};
   if (!os) {
+    unsigned diagID = ci.diagnostics().getCustomDiagID(
+        clang::DiagnosticsEngine::Error, "failed to create the output file");
+    ci.diagnostics().Report(diagID);
     return;
   }
 
   mlirModule.print(*os);
-  }
+}
 
 void EmitObjAction::ExecuteAction() {
   CompilerInstance &ci = this->instance();

flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp

@@ -32,10 +32,8 @@ static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
     return std::make_unique<PrintPreprocessedAction>();
   case ParseSyntaxOnly:
     return std::make_unique<ParseSyntaxOnlyAction>();
-    break;
   case EmitFir:
     return std::make_unique<EmitFirAction>();
-    break;
   case EmitObj:
     return std::make_unique<EmitObjAction>();
   case DebugUnparse:

flang/test/Driver/emit_fir.f90 renamed to flang/test/Driver/emit-fir.f90

@@ -161,4 +161,30 @@ TEST_F(FrontendActionTest, ParseSyntaxOnly) {
           .contains(
               ":1:14: error: IF statement is not allowed in IF statement\n"));
 }
+
+TEST_F(FrontendActionTest, EmitFir) {
+  // Populate the input file with the pre-defined input and flush it.
+  *(inputFileOs_) << "end program";
+  inputFileOs_.reset();
+
+  // Set-up the action kind.
+  compInst_.invocation().frontendOpts().programAction = EmitFir;
+  compInst_.invocation().preprocessorOpts().noReformat = true;
+
+  // Set-up the output stream. We are using output buffer wrapped as an output
+  // stream, as opposed to an actual file (or a file descriptor).
+  llvm::SmallVector<char, 256> outputFileBuffer;
+  std::unique_ptr<llvm::raw_pwrite_stream> outputFileStream(
+      new llvm::raw_svector_ostream(outputFileBuffer));
+  compInst_.set_outputStream(std::move(outputFileStream));
+
+  // Execute the action.
+  bool success = ExecuteCompilerInvocation(&compInst_);
+
+  // Validate the expected output.
+  EXPECT_TRUE(success);
+  EXPECT_TRUE(!outputFileBuffer.empty());
+  EXPECT_TRUE(
+      llvm::StringRef(outputFileBuffer.data()).contains("func @_QQmain"));
+}
 } // namespace