[HLSL] Add the Frs DXC driver option (#157690)

This pr adds `Frs` as a `DXC` driver option.

It is done by invoking `llvm-objcopy` with the `extract-section=RTS0`
argument specified to output the separate `DXContainer`.

Option behaviour as a reference is found
[here](https://github.com/llvm/wg-hlsl/blob/main/proposals/0029-root-signature-driver-options.md#option-frs).

This resolves: #150277.

Author: inbelic

clang/include/clang/Driver/Options.td

@@ -9418,6 +9418,8 @@ def dxc_Fo : DXCJoinedOrSeparate<"Fo">,
   HelpText<"Output object file">;
 def dxc_Fc : DXCJoinedOrSeparate<"Fc">,
   HelpText<"Output assembly listing file">;
+def dxc_Frs : DXCJoinedOrSeparate<"Frs">,
+  HelpText<"Output additional root signature object file">;
 def dxil_validator_version : Option<["/", "-"], "validator-version", KIND_SEPARATE>,
   Group<dxc_Group>, Flags<[HelpHidden]>,
   Visibility<[DXCOption, ClangOption, CC1Option]>,

clang/lib/Driver/ToolChains/HLSL.cpp

@@ -337,7 +337,13 @@ void tools::hlsl::LLVMObjcopy::ConstructJob(Compilation &C, const JobAction &JA,
   CmdArgs.push_back(Output.getFilename());
 
   if (Args.hasArg(options::OPT_dxc_strip_rootsignature)) {
-    const char *Frs = Args.MakeArgString("--remove-section=RTS0");
+    const char *StripRS = Args.MakeArgString("--remove-section=RTS0");
+    CmdArgs.push_back(StripRS);
+  }
+
+  if (Arg *Arg = Args.getLastArg(options::OPT_dxc_Frs)) {
+    const char *Frs =
+        Args.MakeArgString("--extract-section=RTS0=" + Twine(Arg->getValue()));
     CmdArgs.push_back(Frs);
   }
 
@@ -524,7 +530,7 @@ bool HLSLToolChain::requiresBinaryTranslation(DerivedArgList &Args) const {
 bool HLSLToolChain::requiresObjcopy(DerivedArgList &Args) const {
   return Args.hasArg(options::OPT_dxc_Fo) &&
          (Args.hasArg(options::OPT_dxc_strip_rootsignature) ||
-          isRootSignatureTarget(Args));
+          Args.hasArg(options::OPT_dxc_Frs) || isRootSignatureTarget(Args));
 }
 
 bool HLSLToolChain::isLastJob(DerivedArgList &Args,

clang/test/Driver/dxc_frs.hlsl

@@ -0,0 +1,10 @@
+// RUN: %clang_dxc -T cs_6_0 /Fo %t.dxo /Frs %t.rs.dxo -### %s 2>&1 | FileCheck %s
+
+// Test to demonstrate extracting the root signature to the specified
+// output file with /Frs.
+
+// CHECK: "{{.*}}llvm-objcopy{{(.exe)?}}" "{{.*}}.obj" "{{.*}}.dxo" "--extract-section=RTS0={{.*}}.rs.dxo"
+
+[shader("compute"), RootSignature("")]
+[numthreads(1,1,1)]
+void EmptyEntry() {}