[clang] Enable AST mutation in the constant evaluator (PR #115168)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> @zygoloid @katzdm for awareness

This PR is still in flux. Discussions are lengthy and typically happen in meetings.
My plan is to get to a stable point, write down how we got there, alternatives rejected (with reasoning), and get the PR out of the draft status. This would be a good point to engage for people who are interested, but not interested enough to attend a lot of meetings.

https://github.com/llvm/llvm-project/pull/115168