Fix ASTReader::FinishedDeserializing().

We were passing a decl to the consumer after all pending deserializations were finished
but this was not enough; due to processing by the consumer we may end up into yet another
deserialization process but the way FinishedDeserializing() was setup we would not ensure
that everything was fully deserialized before returning to the consumer.

Separate ASTReader::FinishedDeserializing() into two semantic actions.
The first is ensuring that a deserialization process ends up will fully deserialized decls/types even
if the process is started by the consumer.
The second is pushing "interesting" decls to the consumer; we make sure that we don't re-enter this
section recursively be checking a variable.

llvm-svn: 150160