[clang] [HLSL][SPIRV] Add CLI option `-fspv-extension` (PR #137985)

[Cassandra Beckley via cfe-commits]

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@@ -9213,6 +9213,11 @@ def metal : DXCFlag<"metal">, HelpText<"Generate Metal library">;
 def fspv_target_env_EQ : Joined<["-"], "fspv-target-env=">, Group<dxc_Group>,
   HelpText<"Specify the target environment">,
   Values<"vulkan1.2, vulkan1.3">;
+def fspv_extension_EQ
+    : Joined<["-"], "fspv-extension=">,
+      Group<dxc_Group>,
+      HelpText<"Specify the available SPIR-V extensions. If this option is not "
+               "specificed, then all extensions are available.">;
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cassiebeckley wrote:

nit:

```suggestion
               "specified, then all extensions are available.">;
```

https://github.com/llvm/llvm-project/pull/137985