This allows us to map back from a type to the declaration in the dump,
so that we can perform more fine-grained analysis like whether a string
has been changed to string enum.
When we compare the APIs of entire SDKs, we can assume super classes
are always present in the API dump. However, this assumption is invalid
for framework-level API diff checking, where super classes can come from
other modules whose contents are not serialized.
rdar://33110442
In Jenkins bot, we've seen "exception std::length_error: vector" for
api-digester tests. Tentatively, this patch initializes a major vector with a
reasonably large size, which may avoid its wild capacity growth.
