[clang] [llvm] [SPIRV][HLSL] Add DXC compatibility option for extension (PR #151554)

[Chris B via llvm-commits]

================
@@ -9428,8 +9428,12 @@ def fspv_target_env_EQ : Joined<["-"], "fspv-target-env=">, Group<dxc_Group>,
 def fspv_extension_EQ
     : Joined<["-"], "fspv-extension=">,
       Group<dxc_Group>,
-      HelpText<"Specify the available SPIR-V extensions. If this option is not "
-               "specified, then all extensions are available.">;
+      HelpText<
+          "Specify the available SPIR-V extensions. If this option is not "
+          "specified, then all extensions are available. If KHR is specified, "
+          "then all KHR extensions will be available. If DXC is specified, "
+          "then all extensions implemented by the DirectXShader compiler will "
----------------
llvm-beanz wrote:

nit: Generally we write this either as 3 separate words or all as one word.
```suggestion
          "then all extensions implemented by the DirectX Shader Compiler will "
```

https://github.com/llvm/llvm-project/pull/151554