This PR adds support for `-ffine-grained-bitfield-accesses`. I reused
the tests from classic CodeGen, available here:
[c2c881fcc8/clang/test/CodeGenCXX/finegrain-bitfield-access.cpp)
We produce almost exactly the same codegen, except when returning a
variable: we emit an extra variable to hold the return value, whereas
classic CodeGen does not. Also, the GEP instructions use slightly
different syntax compared to classic CodeGen.
