Reserve AllDelayed vector to upper bound.
File: [swift/lib/Parse/PersistentParserState.cpp]
Observing this code snipped closely in function, [void PersistentParserState::parseAllDelayedDeclLists()]:
```
for (auto &P: DelayedDeclListStates) {
AllDelayed.push_back(P.first);
}
```
We observe that 'AllDelayed' gets maximum of DelayedDeclListStates.size() elements pushed into it.
So it is better to reserve 'vector AllDelayed' to that max size, to save reallocation cost.
I believe we can use size() on [DelayedDeclListStates], which is of type: ```llvm::DenseMap<IterableDeclContext *, std::unique_ptr<DelayedDeclListState>>``` because of the ```DenseMapIterator``` implementation of size().
[http://llvm.org/doxygen/DenseMap_8h_source.html#l00126]
