[clang] [llvm] [NFC][HLSL][DirectX] Let `HLSLRootSignature` reuse the `dxbc` defined enums (PR #145986)

[Justin Bogner via cfe-commits]

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@@ -127,6 +127,7 @@ TEST_F(ParseHLSLRootSignatureTest, ValidParseEmptyTest) {
 }
 
 TEST_F(ParseHLSLRootSignatureTest, ValidParseDTClausesTest) {
+  using FlagT = llvm::dxbc::DescriptorRangeFlags;
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bogner wrote:

I wouldn't rename this to `FlagT`, but instead just introduce the `DescriptorRangeFlags` name to the scope. That is:
```suggestion
  using llvm::dxbc::DescriptorRangeFlags;
```

Then we can use `DescriptorRangeFlags::DataStaticWhileSetAtExecute` and the like without explicitly writing out the `llvm::dxbc::` namespace every time, which improves readability and in this case actually simplifies the diff quite a bit.

https://github.com/llvm/llvm-project/pull/145986