[CIR][CodeGen][Builtin] `mempcpy` and `__builtin_mempcpy`

[tommymcm]

Minimized test cases:

void test(void *a, void *b) { 
  mempcpy(a, b, 0); 
}

void test(void *a, void *b) {
  __builtin_mempcpy(a, b, 0);
}

Failed assertion:

clangir/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp

Line 1691 in aeac352

llvm_unreachable("mempcpy is NYI");

Flags: -fclangir -O2

[badumbatish]

godbolt link for builtin

https://godbolt.org/z/e1hdMGMaf