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

[tommymcm]

Minimized test case:

a(A) { mempcpy(A, A, 0); }

Failed assertion:

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

Line 1691 in aeac352

llvm_unreachable("mempcpy is NYI");

Flags: -fclangir -O2