Update to CIRCT 1.15.0

[youngar]

This PR updates the version of CIRCT.
The command line options for firtool command have been updated.
CIRCT release notes here: https://github.com/llvm/circt/releases/tag/sifive%2F1%2F15%2F0

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• bug fix
• performance improvement
• backend code generation

API Impact

• Changes the required version of CIRCT.

Backend Code Generation Impact

• Split large variadic operations into expressions, improving VCS compile time

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[CLAassistant]

All committers have signed the CLA.

[jackkoenig]

Changes like this cause havoc on the current branch protection rules (because it changes the name of the matrix tests). I have #2743 which should fix that to where we can change branch protection to only point to all-tests-passed again.