[Clang][LoongArch] Fix ABI handling of empty structs in C++ to match GCC behaviour

GCC doesn't ignore non-zero-length array of empty structures in C++
while clang does. What this patch did is to match GCC's behaviour
although this rule is not documented in psABI.

Similar to D142327 for RISCV.

Reviewed By: xry111, xen0n

Differential Revision: https://reviews.llvm.org/D156116