[clang] [clang][bytecode] Implement support for `Expr::EvaluateWithSubstitution()` (PR #204781)

[Alexis Engelke via cfe-commits]

aengelke wrote:

I don't know, I do ~zero work on Clang. I just saw that somewhat substantial bump on c-t-t and wanted to point this out. (And looking at the size of the visitor, an 8kiB vtable *seems* to be somewhat large..)

https://github.com/llvm/llvm-project/pull/204781