[CIR] Upstream support for structure types

[andykaylor]

Overview

Upstream the basic support for structure variable types. The initial change should just add support for declaring and initializing global variables that use a simple structure type. Follow-up patches can add support for local variables and accessing structs using the cir.get_member operation.

Suggested minimal test case

// Global
struct S {
  int a;
  int b;
};
S s;

Existing incubator tests

clang/test/CIR/CodeGen/struct.c
(clang/test/CIR/CodeGen/struct.cpp involves class support and should be deferred)

[andykaylor]

I'm starting work on this.