Remove uses of the redundant ".reset(nullptr)" of unique_ptr, in favor of ".reset()"

It's also possible to just write "= nullptr", but there's some question
of whether that's as readable, so I leave it up to authors to pick which
they prefer for now. If we want to discuss standardizing on one or the
other, we can do that at some point in the future.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213439 91177308-0d34-0410-b5e6-96231b3b80d8