[clang] [llvm] [HLSL] Implement the `fwidth` intrinsic for DXIL and SPIR-V target (PR #161378)

[Farzon Lotfi via cfe-commits]

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@@ -21,3 +21,4 @@ def subgroup_local_invocation_id : SPIRVBuiltin<"uint32_t()", [NoThrow, Const]>;
 def distance : SPIRVBuiltin<"void(...)", [NoThrow, Const]>;
 def length : SPIRVBuiltin<"void(...)", [NoThrow, Const]>;
 def smoothstep : SPIRVBuiltin<"void(...)", [NoThrow, Const, CustomTypeChecking]>;
+def fwidth : SPIRVBuiltin<"void(...)", [NoThrow, Const, CustomTypeChecking]>;
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farzonl wrote:

we only put things here if this builtin should work for both CL and VK contexts. If this is just a VK feature please move to 
`clang/include/clang/Basic/BuiltinsSPIRVVK.td`

https://github.com/llvm/llvm-project/pull/161378