[analyzer] Account for the "interesting values" hash table resizing.

RegionStoreManager::getInterestingValues() returns a pointer to a
std::vector that lives inside a DenseMap, which is constructed on demand.
However, constructing one such value can lead to constructing another
value, which will invalidate the reference created earlier.

Fixed by delaying the new entry creation until the function returns.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175582 91177308-0d34-0410-b5e6-96231b3b80d8