[clang] [clang][NFCI] Clarify ownership of PragmaHandlers (PR #117703)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Unless I'm missing something, this doesn't really make sense.  Pragma handlers being owned by the Parser makes perfect sense to me, and making them shared ownership does not make sense.

'Raw' pointers as non-owned references to a unique-ptr are completely sensible, so IMO, it is not a problem.  Can you share what the Static assert complaint is that makes you think we should be doing this?

https://github.com/llvm/llvm-project/pull/117703