Fix unnecessary transmute and unnecessary unsafe block warnings on bitfield codegen

[armoha]

Fixes #2807 , Fixes #3355 (=#3241 (comment))

PR #3335 replaced transmute with as _ in the union branch getter/setter and the non-union setter, and removed the unsafe block from the union branch. It left non-union getter and raw getter, emitting unsafe { transmute(...) } and triggering unnecessary_transmutes warning for integer (#2807). It replaced transmute with val as _ for non-union setter and kept unsafe block which is required for Rust unions (its field access is unsafe) but redundant for the rest of cases (#3355).

This PR type-dispatches the non-union getter codegen, and removes unnecessary unsafe block from the non-union setter. It dispatches on bitfield_ty.canonical_type(ctx).kind() instead of bitfield_ty.kind() to resolve Alias/Elaborated wrappers clang applies.

• integer getter: unsafe { transmute(get_const() as u32) } (unnecessary transmutes) -> get_const() as u32 as _
• bool getter: unsafe { transmute(get_const() as u8) } -> get_const() as u8 != 0
• enum getter: unchanged. unsafe { transmute(get_const() as u32) }
• non-union setter on Rust unions: unchanged. unsafe { let val: u32 = val as _; self._bitfield_1.set_const(val as u64) }
• non-union setter for the rest of cases: removes unnecessary unsafe block for regular structs. let val: u32 = val as _; self._bitfield_1.set_const(val as u64)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[armoha]

header_issue_544_stylo_creduce_2_hpp and nsBaseHashtable tests were overwritten accidentally, probably due to LLVM versions and AST output mismatch on template parameter. (I tested with clang version 21.1.8)

r? @emilio