[clangd][HLSL] RootSignature Hover Leaking Internal Identifier

## Expected behavior

Hovering over `[RootSignature("RS_CBV")]` should display the documentation for the` RootSignature` attribute without exposing internal compiler details.

## Observed behavior

Hover shows the documentation but also leaks the definition.

## Reproduction

```hlsl
#define RS_CBV "CBV(b0)"

[RootSignature(RS_CBV)]
[numthreads(1, 1, 1)]
void CS_ValidCBV(uint3 tid : SV_DispatchThreadID) {}
```
Hovering over `RootSignature` shows the right documentation, but also shows something like `[RootSignature("__hlsl_rootsig_decl_5043168244180965862")]`.

## Root cause
The HLSL parser processes the string literal argument of `RootSignature` and produces an internal `IdentifierInfo` with a generated name. This generated identifier is stored in `RootSignatureAttr` via `getSignatureIdent()`. The original string literal is discarded after parsing and not stored in the attribute. The generic hover path calls `A->printPretty()`, which prints the stored identifier.

## Proposed fix
Add a `dyn_cast` branch in `getHoverContents(const Attr *A)` that sets `HI.Name` and `HI.Documentation` directly, skipping `printPretty`.

Start at getHoverContents(const Attr *A) and inspect the existing generic hover handling for attributes, along with RootSignatureAttr and getSignatureIdent(). Add the RootSignature-specific hover behavior described in the issue. Done means RootSignature hover still shows its documentation without exposing the generated internal identifier.

Written by the indexing model from the issue text.