[clang][Driver] Support passing arbitrary args to -cc1as with -Xclangas.

Unlike the -Xassembler idea mentioned in #97517, this one comes with no GCC
compatibility concerns, and simply mirrors what -Xclang does for -cc1.

This is useful for mostly the same reasons that -Xclang is. The motivating use
case is zig cc, however, where we use -Xclang -target-feature to pass the
exhaustive list of target features to Clang for C-family files. Before this
commit, there was no way to do the same for assembly files.

For context, Zig bases all of its target feature info directly on the info in
LLVM's backends, including all the dependency relationships therein. So it just
makes more sense for Zig to be able to directly pass all this info to the
assembler, rather than having to manually reconstruct the corresponding frontend
command line flags for every target.

Closes #97517.

Author: alexrp

clang/docs/UsersManual.rst

@@ -5146,6 +5146,7 @@ Execute ``clang-cl /?`` to see a list of supported options:
       -v                      Show commands to run and use verbose output
       -W<warning>             Enable the specified warning
       -Xclang <arg>           Pass <arg> to the clang compiler
+      -Xclangas <arg>         Pass <arg> to the clang assembler
 
 The /clang: Option
 ^^^^^^^^^^^^^^^^^^

clang/include/clang/Driver/Options.td

@@ -958,10 +958,18 @@ def Xclang : Separate<["-"], "Xclang">,
   HelpText<"Pass <arg> to clang -cc1">, MetaVarName<"<arg>">,
   Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
   Group<CompileOnly_Group>;
+def Xclangas : Separate<["-"], "Xclangas">,
+  HelpText<"Pass <arg> to clang -cc1as">, MetaVarName<"<arg>">,
+  Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
+  Group<CompileOnly_Group>;
 def : Joined<["-"], "Xclang=">, Group<CompileOnly_Group>,
   Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
   Alias<Xclang>,
   HelpText<"Alias for -Xclang">, MetaVarName<"<arg>">;
+def : Joined<["-"], "Xclangas=">, Group<CompileOnly_Group>,
+  Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
+  Alias<Xclangas>,
+  HelpText<"Alias for -Xclangas">, MetaVarName<"<arg>">;
 def Xcuda_fatbinary : Separate<["-"], "Xcuda-fatbinary">,
   HelpText<"Pass <arg> to fatbinary invocation">, MetaVarName<"<arg>">;
 def Xcuda_ptxas : Separate<["-"], "Xcuda-ptxas">,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -8895,6 +8895,12 @@ void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,
   CollectArgsForIntegratedAssembler(C, Args, CmdArgs,
                                     getToolChain().getDriver());
 
+  // Forward -Xclangas arguments to -cc1as
+  for (auto Arg : Args.filtered(options::OPT_Xclangas)) {
+    Arg->claim();
+    CmdArgs.push_back(Arg->getValue());
+  }
+
   Args.AddAllArgs(CmdArgs, options::OPT_mllvm);
 
   if (DebugInfoKind > llvm::codegenoptions::NoDebugInfo && Output.isFilename())

clang/test/Driver/Xclangas.s

@@ -0,0 +1,4 @@
+/// Check that -Xclangas/-Xclangas= are passed to -cc1as.
+// RUN: %clang -### -Werror -Xclangas -target-feature -Xclangas=+v5t %s 2>&1 | FileCheck %s
+// CHECK: -cc1as
+// CHECK-SAME: "-target-feature" "+v5t"