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

[Finn Plummer via llvm-commits]

================
@@ -404,7 +417,7 @@ TEST_F(ParseHLSLRootSignatureTest, ValidSamplerFlagsTest) {
   ASSERT_TRUE(std::holds_alternative<DescriptorTableClause>(Elem));
   ASSERT_EQ(std::get<DescriptorTableClause>(Elem).Type, ClauseType::Sampler);
   ASSERT_EQ(std::get<DescriptorTableClause>(Elem).Flags,
-            DescriptorRangeFlags::ValidSamplerFlags);
----------------
inbelic wrote:

It previously followed the same pattern from DXC to define the valid flags as a separate enum in `DescriptorRangeFlags`. However, we decided not to continue that pattern.

`ValidSamplerFlags` was previously set to `DescriptorsVolatile`, so they are equivalent.

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