[CIR] Fix bitfield store locations for assignment codegen (#184005)
Update bitfield-assignment codegen to emit stores at
assignment-expression source locations.
Keep `cir.set_bitfield` aligned with other store-like operations.
Prevent regressions that reattach bitfield stores to declaration-site
locations.
Add a CIR test on `clang/test/CIR/CodeGen/bitfield-assignment-loc.c`.
Fix https://github.com/llvm/llvm-project/issues/183759