[clang] Include the error message in file reading error diagnostic

in order to provide as much information as possible to the user. The
diagnostic will now look like for example:

  error: error reading '/tmp/foo.c': Permission denied

(This addresses a FIXME from 2019, 9ef6c49)

Differential revision: https://reviews.llvm.org/D146280

Author: zmodem

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -11,7 +11,7 @@ class BackendInfo : CatBackend, ShowInSystemHeader;
 let Component = "Frontend" in {
 
 def err_fe_error_opening : Error<"error opening '%0': %1">;
-def err_fe_error_reading : Error<"error reading '%0'">;
+def err_fe_error_reading : Error<"error reading '%0': %1">;
 def err_fe_error_reading_stdin : Error<"error reading stdin: %0">;
 def err_fe_error_backend : Error<"error in backend: %0">, DefaultFatal;
 

clang/lib/Frontend/CompilerInstance.cpp

@@ -981,10 +981,9 @@ bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input,
                        ? FileMgr.getSTDIN()
                        : FileMgr.getFileRef(InputFile, /*OpenFile=*/true);
   if (!FileOrErr) {
-    // FIXME: include the error in the diagnostic even when it's not stdin.
     auto EC = llvm::errorToErrorCode(FileOrErr.takeError());
     if (InputFile != "-")
-      Diags.Report(diag::err_fe_error_reading) << InputFile;
+      Diags.Report(diag::err_fe_error_reading) << InputFile << EC.message();
     else
       Diags.Report(diag::err_fe_error_reading_stdin) << EC.message();
     return false;

clang/tools/driver/cc1as_main.cpp

@@ -384,8 +384,8 @@ static bool ExecuteAssemblerImpl(AssemblerInvocation &Opts,
       MemoryBuffer::getFileOrSTDIN(Opts.InputFile, /*IsText=*/true);
 
   if (std::error_code EC = Buffer.getError()) {
-    Error = EC.message();
-    return Diags.Report(diag::err_fe_error_reading) << Opts.InputFile;
+    return Diags.Report(diag::err_fe_error_reading)
+           << Opts.InputFile << EC.message();
   }
 
   SourceMgr SrcMgr;

flang/lib/Frontend/FrontendAction.cpp

@@ -48,7 +48,7 @@ bool FrontendAction::beginSourceFile(CompilerInstance &ci,
       unsigned diagID;
       if (llvm::vfs::getRealFileSystem()->exists(input.getFile())) {
         ci.getDiagnostics().Report(clang::diag::err_fe_error_reading)
-            << input.getFile();
+            << input.getFile() << "not a regular file";
         diagID = ci.getDiagnostics().getCustomDiagID(
             clang::DiagnosticsEngine::Error, "%0 is not a regular file");
       } else {