[CIR] Upstream binary expression support

[andykaylor]

Overview

Upstream support for binary expressions and the cir.binop operation. The initial change should just add support a single operation, such as assignment. Other binary operators can then be added afterwards.

Suggested minimal test case

void f(int i) {
  i = 2;
}

Existing incubator tests

clang/test/CIR/CodeGen/binop.cpp (could use improvement)

[andykaylor]

@mmha is going to look at this after he's finished with cast operations.