[CIR] Upstream handling for __builtin_prefetch

[andykaylor]

Overview

Upstream handling for the __builtin_prefetch function, including implemetation and lowering of the cir.prefetch operation.

Suggested minimal test case

void foo(void *a) {
  __builtin_prefetch(a, 1, 1);
}

Existing incubator tests

clang/test/CIR/CodeGen/builtin-prefetch.c

[andykaylor]

@kimsh02 This would be similar to the ptr diff work you did.

[kimsh02]

@andykaylor Alright, thanks! I'll take it up