mach: Restructure exception server to separate protected exception cases

In contrast to to the modern mach_exc, the original 32-bit exc does not
support identity-protected exceptions (e.g.,
EXCEPTION_IDENTITY_PROTECTED). Under the previous unified switch
statement design, the protected cases in ExcServer<Traits>::
MachMessageServerFunction were compiled (but dead) when Traits =
ExcTraits. This forced the use of raw pointer type casts to compile
mismatched exception code arrays (casting 64-bit exception codes to
32-bit exception codes), which was flagged during spanification and by
UBSan.

This change restructures the MachMessageServerFunction by splitting it
into two distinct switch statements:
- A switch statement for standard exceptions compiled for all Traits.
- A switch statement for identity-protected exceptions nested inside
  a single `if constexpr (std::is_same_v<Traits, MachExcTraits>)` block.

This ensures protected exception cases are only compiled and
instantiated when Traits = MachExcTraits.

Bug: 401232341
Change-Id: I1d4ae2d46d91d87aa67d5a5a5e445d1f9e2dc067
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/7880932
Commit-Queue: Justin Cohen <justincohen@google.com>
Reviewed-by: Mark Mentovai <mark@chromium.org>