[Clang][LoongArch] Implement ABI lowering

Reuse most of RISCV's implementation with several exceptions:

1. Assign signext/zeroext attribute to args passed in stack.
On RISCV, integer scalars passed in registers have signext/zeroext
when promoted, but are anyext if passed on the stack. This is defined
in early RISCV ABI specification. But after this change [1], integers
should also be signext/zeroext if passed on the stack. So I think
RISCV's ABI lowering should be updated [2].

While in LoongArch ABI spec, we can see that integer scalars narrower
than GRLEN bits are zero/sign-extended no matter passed in registers
or on the stack.

2. Zero-width bit fields are ignored.
This matches GCC's behavior but it hasn't been documented in ABI sepc.
See https://gcc.gnu.org/r12-8294.

3. `char` is signed by default.
There is another difference worth mentioning is that `char` is signed
by default on LoongArch while it is unsigned on RISCV.

This patch also adds `_BitInt` type support to LoongArch and handle it
in LoongArchABIInfo::classifyArgumentType.

[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/cec39a064ee0e5b0129973fffab7e3ad1710498f
[2] https://github.com/llvm/llvm-project/issues/57261

Differential Revision: https://reviews.llvm.org/D132285