refactor(parse): cache "expression list is a fold expression"

[JohelEGP]

This work arounds llvm/llvm-project#73336.
This is the delta when I compile cppfront with LLVM 18 at -O2:

real	3m10.558s
user	3m8.552s
sys	0m0.474s

real	0m46.694s
user	0m45.891s
sys	0m0.362s

[hsutter]

Ah, that makes sense for breaking the dependency -- cache it right before returning the successful parse, since everything is immutable after that. Thanks!