Add tests using lit as a test harness

[cbezault]

Description

This PR adds test coverage. Much of the python code in this PR is based off of, or taken verbatim from, code found in llvm-project/libcxx/test.

After configuring and building, running ctest from the build root will run the libcxx and std tests.

Subsets of the tests can be run using the llvm-lit.py script found under <build_root>/tests/llvm-lit/lit.py.
For example, python3.8 <build_root>/tests/llvm-lit/lit.py <source_root>/tests/std/tests/VSO_0000000_any_calling_conventions, will run all tests found under that directory. In the std testsuite there is only a single test under each named directory. In the libcxx testsuite there can be many.

TODO

• Determine if it is worth having an expected_results.txt and how that would interact with the legacy test harness.

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[cbezault]

Some high-level comments.

1. This is still a work in progress as far as cleanup goes. I didn't write a fair amount of the code in this PR and optimistically just took it from libcxx.
2. I have left the llvm headers in every file I took from llvm where the headers were pre-existing. I need to double check they didn't have any files without headers.

Thanks for taking the time to do this first pass review guys. I'll work through your comments.

[barcharcraz]

Some high-level comments.

1. This is still a work in progress as far as cleanup goes. I didn't write a fair amount of the code in this PR and optimistically just took it from libcxx.
2. I have left the llvm headers in every file I took from llvm where the headers were pre-existing. I need to double check they didn't have any files without headers.

Thanks for taking the time to do this first pass review guys. I'll work through your comments.

Yeah, I think a lot of my comments were on things that were pre-existing in llvm code. Depending on how we want to maintain this code and how much we're willing to let it drift from "upstream" we may not want to address all of my concerns :)

[cbezault]

@BillyONeal I think the last thing I need is python3 on the CI vms.

[CaseyCarter]

You seem to have inadvertently moved the LLVM reference back in time.

EDIT: This has been resolved, but since it was the "final" comment in a review, there's no way to mark it as such.

[miscco]

Hooray!

Great work @cbezault 🎉

[miscco]

That said somebody wanted to add descriptions to the Readme.md on how to run the tests locally 😇

[cbezault]

I'm planning on tackling that tomorrow.

[cbezault]

@miscco Didn't get to it today (sorry), dealing with other things and infrastructure instability of the CI. Will be at the top of my TODOs for tomorrow.

[miscco]

@cbezault Dont worry, that was just a friendly banter. I thought working with @CaseyCarter makes you immune to that ;)