[ADT] Add a reserve() method to DenseSet as well as an insert() for R-value

Recommit 278600 with some fixes to make the test more robust.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278604 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed