[LoongArch] Support fastcc and treat it as ccc

As explained in D68559 the `fastcc` calling convention may be requested
under certain conditions, hence the need for supporting it. But unlike
RISCV we actually treat it exactly like ccc, without actually inventing
any performance hack right here. And CSKY does the same thing.

This is going to fix a few more test cases with native LoongArch builds.

Differential Revision: https://reviews.llvm.org/D134443