Unify duplicated bool type transforms into a shared helper

[tannergooding]

The bool/bool* type-transform logic used when emitting P/Invoke-facing signatures was copy-pasted across three sites in PInvokeGenerator.VisitDecl.cs. The plain bool -> byte remap, the compat-mode bool* -> byte* remap, and the nativeTypeName fallback were each duplicated, so keeping the sites in sync was manual and drift-prone.

This factors that logic into a single private helper and calls it from each site:

private void TransformBoolType(ref string typeName, ref string nativeTypeName)

Sites unified (all in PInvokeGenerator.VisitDecl.cs):

• VisitFieldDecl
• VisitFunctionDecl virtual / no-body return handling
• the anonymous-record field emit path

The anonymous-record site previously discarded nativeTypeName via out _; it now captures it so it can pass the ref. That field emits NativeTypeName = null regardless, so the discarded value stays byte-identical.

This is behavior-preserving; generated output is unchanged.

Deferred intentionally: the four bool-only sites in PInvokeGenerator.TypeResolution.cs are left as-is. They only do bool -> byte and deliberately omit the bool* transform; two of them build delegate* signatures, so folding them into the full helper would change fnptr/template codegen. The != 0 write in VisitDecl is a value expression rather than a type transform, so it is out of scope.

Validation: Release build with 0 warnings / 0 errors, and all 3708 golden-file generator tests pass (byte-identical output).