[GWP-ASan] Add version header.

Adds magic version header to AllocatorState. This can be used by
out-of-process crash handlers, like Crashpad on Fuchsia, to do offline
reconstruction of GWP-ASan crash metadata.

Crashpad on Fuchsia is intending on dumping the AllocationMetadata pool
and the AllocatorState directly into the minidump. Then, using the
version number, they can unpack the data on serverside using a versioned
unpack tool.

Also add some asserts to make sure the version number gets bumped if the
internal structs get changed.

Reviewed By: eugenis, mcgrathr

Differential Revision: https://reviews.llvm.org/D106690