Add a unit test for 'swap', and fix a pile of bugs in

SmallDenseMap::swap.

First, make it parse cleanly. Yay for uninstantiated methods.

Second, make the inline-buckets case work correctly. This is way
trickier than it should be due to the uninitialized values in empty and
tombstone buckets.

Finally fix a few typos that caused construction/destruction mismatches
in the counting unittest.

llvm-svn: 158641