[flang][driver] Use -Xflang in diagnostics

When an option that is only available in `flang -fc1` is provided to
`flang`, emit a diagnostic with a suggestion containing "did you mean
-Xflang '-foo'".

Partially addresses llvm#163550.

Author: tarunprabhu

clang/lib/Driver/Driver.cpp

@@ -308,9 +308,18 @@ InputArgList Driver::ParseArgStrings(ArrayRef<const char *> ArgStrings,
     auto ArgString = A->getAsString(Args);
     std::string Nearest;
     if (getOpts().findNearest(ArgString, Nearest, VisibilityMask) > 1) {
-      if (!IsCLMode() &&
-          getOpts().findExact(ArgString, Nearest,
-                              llvm::opt::Visibility(options::CC1Option))) {
+      if (IsFlangMode()) {
+        if (getOpts().findExact(ArgString, Nearest,
+                                llvm::opt::Visibility(options::FC1Option))) {
+          DiagID = diag::err_drv_unknown_argument_with_suggestion;
+          Diags.Report(DiagID) << ArgString << "-Xflang " + Nearest;
+        } else {
+          DiagID = diag::err_drv_unknown_argument;
+          Diags.Report(DiagID) << ArgString;
+        }
+      } else if (!IsCLMode() && getOpts().findExact(ArgString, Nearest,
+                                                    llvm::opt::Visibility(
+                                                        options::CC1Option))) {
         DiagID = diag::err_drv_unknown_argument_with_suggestion;
         Diags.Report(DiagID) << ArgString << "-Xclang " + Nearest;
       } else {

flang/test/Driver/flang-f-opts.f90

@@ -1,5 +1,5 @@
-! Test for warnings generated when parsing driver options. You can use this file for relatively small tests and to avoid creating
-! new test files.
+! Test for errors and warnings generated when parsing driver options. You can
+! use this file for relatively small tests and to avoid creating new test files.
 
 ! RUN: %flang -### -S -O4 -ffp-contract=on %s 2>&1 | FileCheck %s
 
@@ -26,3 +26,20 @@
 ! RUN:     | FileCheck %s -check-prefix=WARN-BUILTIN-MULTIPLE
 ! WARN-BUILTIN-MULTIPLE: warning: '-fbuiltin' is not valid for Fortran
 ! WARN-BUILTIN-MULTIPLE: warning: '-fno-builtin' is not valid for Fortran
+
+! When emitting an error with a suggestion, ensure that the diagnostic message
+! uses '-Xflang' instead of '-Xclang'. This is typically emitted when an option
+! that is available for `flang -fc1` is passed to `flang`. We use -complex-range
+! since it is only available for fc1. If this option is ever exposed to `flang`,
+! a different option will have to be used in the test below.
+!
+! RUN: not %flang -### -complex-range=full %s 2>&1 \
+! RUN:     | FileCheck %s -check-prefix UNKNOWN-SUGGEST
+!
+! UNKNOWN-SUGGEST: error: unknown argument '-complex-range=full';
+! UNKNOWN-SUGGEST-SAME: did you mean '-Xflang -complex-range=full'
+!
+! RUN: not %flang -### -not-an-option %s 2>&1 \
+! RUN:     | FileCheck %s -check-prefix UNKNOWN-NO-SUGGEST
+!
+! UNKNOWN-NO-SUGGEST: error: unknown argument: '-not-an-option'{{$}}