[clang] [clang] Enable exceptions in CWG2486 test (PR #187195)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> It might be useful to test both with and without exceptions though. ie it's nice to know that this gets diagnosed even when exceptions are disabled

I see a similar test elsewhere with exception disabled:
https://github.com/llvm/llvm-project/blob/073d019c450da7948a23ae240bfc6388971a8f25/clang/test/CXX/conv/conv.fctptr/p1.cpp#L8-L17

Do you think it is enough?

https://github.com/llvm/llvm-project/pull/187195