[NFCI] Change the data structure of MaybeODRUseExprSet

In 1eddce41, I fixed a non-deterministic result problem by switching a
SmallPtrSet to a SmallSetVector to ensure we iterated it
deterministically.  Unfortunately, this seems to show a surprisingly
significant compiletime impact.

This patch does 2 things in an attempt to fix this:

First, it makes the 'small size' optimization 4 instead of 2.  As these
are pointers, this only increases the size of Sema by 4
sizeof(pointer)s (2 for the set, 2 for the vector).

Second, instead of using SmallSetVector, which is a SmallVector +
SmallDenseSet, it uses a SetVector of SmallVector + SmallPtrSet.  The
hope is that the pointer-specific optimizations of the SmallPtrSet will
minimize the impact on compile-time.