[NFC] [Serialization] Use semantical type 'DeclID' for 'CreateDeserialized'

Previously we use 'unsigned' as the type of ID in 'CreateDeserialized'.

And the type of `DeclID` in serialization is 'uint32_t', so there is
minor inconsistency.

Also more importantly, if we want to extend the type of DeclID from
uint32_t to uint64_t, we may be in trouble due to we forgot updating the
a lot of 'CreateDeserialized'.

So this patch tries to use semantical type 'DeclID' for
'*Decl::CreateDeserialized' to make sure it is tightly consistent.