[clang][modules] Avoid serializing all diag mappings in non-deterministic order

When writing a pcm, we serialize diagnostic mappings in order to
accurately reproduce the diagnostic environment inside any headers from
that module. However, the diagnostic state mapping table contains
entries for every diagnostic ID ever accessed, while we only want to
serialize the ones that are actually modified from their default value.
Futher, we need to serialize them in a deterministic order.

rdar://111477511

Differential Revision: https://reviews.llvm.org/D154016