LLVM 13 Updates

[nickpdemarco]

Description

This change builds off of @fosterbrereton's branch fosterbrereton/llvm13-updates.

1. Update CMakeLists.txt to use a more standard brew command to locate llvm.
2. Update .gitignore to ignore build*/
3. Add a few header includes related to a boost::filesystem change.

Related Issue

Motivation and Context

Required to build with LLVM 13.

How Has This Been Tested?

Tested by running a ninja-powered build on my M1 laptop.

Screenshots (if appropriate):

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X] I have signed the Adobe Open Source CLA.
• [X] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [X] All new and existing tests passed.

[jaredwy]

Looks good to me, just one question. Let me know if you want me to tets that for you.

[nickpdemarco]

@jaredwy Thanks for the suggestion to test multiple versions. It works as expected on my machine with multiple versions of LLVM installed. If you're satisfied, please merge at your earliest convenience.