[CIR] Upstream handling of __builtin_constant_p

[andykaylor]

Overview

Upstream handling for the __builtin_constant_p function, including implemetation and lowering of the cir.is_constant operation.

Suggested minimal test case

int a = 0;
int f() {
  return __builtin_constant_p(a);
}

Existing incubator tests

clang/test/CIR/CodeGen/builtin-constant-p.c

[siddu0660]

We would like to work on this

[adams381]

I would like to work on this.

[andykaylor]

@siddu0660 I apologize for the disconnect here. I have been helping @adams381 ramp up on upstreaming, starting with implementing the two issues you commented on. I can create a similar issue for you to work on if you like. We have literally thousands of builtin functions that need to be handled.