[HLSL][RootSignature] Plug-in serialization and add full sample testcase (#144769)

This pr extends `dumpRootElements` to invoke the print methods of all
`RootElement`s now that they are all implemented.

Extends the `RootSignatures-AST.hlsl` testcase to have a root element of
each type being parsed, constructed to the in-memory representation mode
and then being dumped as part of the AST dump.

- Update `HLSLRootSignatureUtils.cpp` to extend `dumpRootElements`
- Extend `AST/HLSL/RootSigantures-AST.hlsl` testcase
- Defines the helper `operator<<` for `RootElement`
- Small correction to the output of `numDescriptors` to be `unbounded`
in special case

Resolves https://github.com/llvm/llvm-project/issues/124595.

GitOrigin-RevId: 310a62f88aa9610b125f7958a131f3d76df50849