Support LLVM 16, GCC 11 and MSVC 2022 17.6

[dpaoliello]

• Updated code, build system and test to support LLVM 16.
• Updated tests to work with GCC 11.
• Fixed issues with targeting MSVC, and updated tests to use MSVC on Windows.
• Added an "atomics" library for Windows that utilized C11 atomics.
• Fixed build system to support building LLVM and llvm-cbe at the same time (i.e., setting the correct dependencies and marking CBackend as a component group).

Testing:

• Ubuntu 22.04 + GCC 11 + LLVM 16
• Windows 11 + MSVC 2022 17.6 + LLVM 16

Note: I have not tested this against LLVM 10 - it should still compile, but the tests now make the assumption that we are using LLVM 16 (namely I'm setting options to disable opaque pointers).

[vtjnash]

I have a general policy against supporting multiple versions by adding LLVM version check ifdef code anyways (https://github.com/JuliaHubOSS/llvm-cbe/issues/60#issue-548626981), so looks good to me, and thanks for the PR!

[vtjnash]

Switching to opaque pointers would be the next big step. My guess is that it would be pretty minimal changes required to change all pointee types to void, except when they are required by operation itself, in which cases LLVM still has them (https://github.com/JuliaHubOSS/llvm-cbe/issues/154)

[dpaoliello]

Switching to opaque pointers would be the next big step. My guess is that it would be pretty minimal changes required to change all pointee types to void, except when they are required by operation itself, in which cases LLVM still has them (#154)

Yep, I'm working on that right now.