[clang] [clang] Fix a use-after-free in expression evaluation (PR #118480)

[kadir çetinkaya via cfe-commits]

kadircet wrote:

still trying to come up with a reproducer.

i am also not sure if this is the best place to have the cleanup, but if i did that closer to [leaves](https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/ExprConstant.cpp#L16376-L16385) tests start failing. so open for suggestions here :)

https://github.com/llvm/llvm-project/pull/118480