[flang][Driver] Add -masm option to flang (#81490)

The motivation here was a suggestion over in Compiler Explorer. You can
use `-mllvm` already to do this but since gfortran supports `-masm`, I
figured I'd try to add it.

This is done by flang expanding `-masm` into `-mllvm x86-asm-syntax=`,
then passing that to fc1. Which then collects all the `-mllvm` options
and forwards them on.

The code to expand it comes from clang `Clang::AddX86TargetArgs` (there
are some other places doing the same thing too). However I've removed
the `-inline-asm` that clang adds, as fortran doesn't have inline
assembly.

So `-masm` for flang purely changes the style of assembly output.

```
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu
<...>
        pushq   %rbp
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu -masm=att
<...>
        pushq   %rbp
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu -masm=intel
<...>
        push    rbp
```

The test is adapted from `clang/test/Driver/masm.c` by removing the
clang-cl related lines and changing the 32 bit triples to 64 bit triples
since flang doesn't support 32 bit targets.

clang/include/clang/Driver/Options.td

@@ -4414,7 +4414,7 @@ def mwatchsimulator_version_min_EQ : Joined<["-"], "mwatchsimulator-version-min=
 def march_EQ : Joined<["-"], "march=">, Group<m_Group>,
   Flags<[TargetSpecific]>, Visibility<[ClangOption, CLOption, DXCOption, FlangOption]>,
   HelpText<"For a list of available architectures for the target use '-mcpu=help'">;
-def masm_EQ : Joined<["-"], "masm=">, Group<m_Group>;
+def masm_EQ : Joined<["-"], "masm=">, Group<m_Group>, Visibility<[ClangOption, FlangOption]>;
 def inline_asm_EQ : Joined<["-"], "inline-asm=">, Group<m_Group>,
   Visibility<[ClangOption, CC1Option]>,
   Values<"att,intel">,

clang/lib/Driver/ToolChains/Flang.cpp

@@ -249,6 +249,20 @@ void Flang::AddRISCVTargetArgs(const ArgList &Args,
   }
 }
 
+void Flang::AddX86_64TargetArgs(const ArgList &Args,
+                                ArgStringList &CmdArgs) const {
+  if (Arg *A = Args.getLastArg(options::OPT_masm_EQ)) {
+    StringRef Value = A->getValue();
+    if (Value == "intel" || Value == "att") {
+      CmdArgs.push_back(Args.MakeArgString("-mllvm"));
+      CmdArgs.push_back(Args.MakeArgString("-x86-asm-syntax=" + Value));
+    } else {
+      getToolChain().getDriver().Diag(diag::err_drv_unsupported_option_argument)
+          << A->getSpelling() << Value;
+    }
+  }
+}
+
 static void addVSDefines(const ToolChain &TC, const ArgList &Args,
                          ArgStringList &CmdArgs) {
 
@@ -374,6 +388,7 @@ void Flang::addTargetOptions(const ArgList &Args,
     break;
   case llvm::Triple::x86_64:
     getTargetFeatures(D, Triple, Args, CmdArgs, /*ForAs*/ false);
+    AddX86_64TargetArgs(Args, CmdArgs);
     break;
   }
 

clang/lib/Driver/ToolChains/Flang.h

@@ -77,6 +77,13 @@ class LLVM_LIBRARY_VISIBILITY Flang : public Tool {
   void AddRISCVTargetArgs(const llvm::opt::ArgList &Args,
                           llvm::opt::ArgStringList &CmdArgs) const;
 
+  /// Add specific options for X86_64 target.
+  ///
+  /// \param [in] Args The list of input driver arguments
+  /// \param [out] CmdArgs The list of output command arguments
+  void AddX86_64TargetArgs(const llvm::opt::ArgList &Args,
+                           llvm::opt::ArgStringList &CmdArgs) const;
+
   /// Extract offload options from the driver arguments and add them to
   /// the command arguments.
   /// \param [in] C The current compilation for the driver invocation

flang/test/Driver/masm.f90

@@ -0,0 +1,10 @@
+! RUN: %flang --target=x86_64-unknown-linux -masm=intel -S %s -### 2>&1 | FileCheck --check-prefix=CHECK-INTEL %s
+! RUN: %flang --target=x86_64-unknown-linux -masm=att -S %s -### 2>&1 | FileCheck --check-prefix=CHECK-ATT %s
+! RUN: not %flang --target=x86_64-unknown-linux -S -masm=somerequired %s -### 2>&1 | FileCheck --check-prefix=CHECK-SOMEREQUIRED %s
+! RUN: %flang --target=aarch64-unknown-eabi -S -masm=intel %s -### 2>&1 | FileCheck --check-prefix=CHECK-AARCH64 %s
+
+! CHECK-INTEL: "-mllvm" "-x86-asm-syntax=intel"
+! CHECK-ATT: "-mllvm" "-x86-asm-syntax=att"
+! CHECK-SOMEREQUIRED: error: unsupported argument 'somerequired' to option '-masm='
+! CHECK-AARCH64: warning: argument unused during compilation: '-masm=intel'
+! CHECK-AARCH64-NOT: -x86-asm-syntax=intel