[CIR] Upstream union support

[andykaylor]

Overview

Upstream support for union types. This will build on the existing record type support, but unions have special layout requirements and need extra handling during lowering to LLVM IR.

Suggested minimal test case

union U {
  int n;
  char c;
};

int f() {
  U u;
  return u.n;
}

Existing incubator tests

clang/test/CIR/CodeGen/union.cpp
clang/test/CIR/CodeGen/union-array.c
clang/test/CIR/CodeGen/union-init.c
clang/test/CIR/CodeGen/union-padding.c

[el-ev]

I would like to take a look into this.