[clangd][HLSL] Code Completion for HLSL Matrix Swizzle Members

## Expected behavior

Typing `m.` on a `float2x2` should suggest `_m00`, `_m01`, `_m10`, `_m11` and `_11`, `_12`, `_21`, `_22`. Typing `m._m00` should continue with `_m` notation only. Typing `m._m00_11` should suggest nothing (mixing notations is invalid).

## Observed behavior
Typing `m.` produced no suggestions.

## Reproduction

```hlsl
[numthreads(1,1,1)]
void main() {
float2x2 m = float2x2(1.0, 2.0, 3.0, 4.0);
float a = m.
}
```

## Root cause
`ConstantMatrixType` was not handled in `CodeCompleteMemberReferenceExpr`. HLSL supports two notation systems (`_m` zero-indexed and `_` one-indexed) that cannot be mixed, but this was not enforced.

## Proposed fix
Add a `ConstantMatrixType` branch in `DoCompletion`. `AddHLSLMatrixSwizzleCompletions` detects the notation from the filter prefix: `starts_with("_m")` locks to 0-indexed (4 chars/component, max 16 chars total), `starts_with("_")` locks to 1-indexed (3 chars/component, max 12 chars total). Mixing notations returns no suggestions.

Start in clangd's CodeCompleteMemberReferenceExpr and follow DoCompletion to see how member-reference completions are dispatched. Inspect the proposed AddHLSLMatrixSwizzleCompletions logic and verify the float2x2 reproduction: both HLSL notations should appear initially, each notation should remain consistent after its prefix, and mixed notation should return no suggestions.

Written by the indexing model from the issue text.