miracleptr: Add RAW_PTR_EXCLUSION for pointers in unions

raw_ptr is a non-trivial type with a destructor. Placing it in a union
deletes the union's default destructor, breaking triviality assumptions
(e.g., std::is_trivially_destructible) and C-style copying (memcpy).
To maintain union triviality and compile-time compatibility, pointers
in unions must remain raw.

Bug: 351867706
Change-Id: Ife9e408e7dd091f2d05c12784aecb78966542134
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1290296
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>