Type parameterize the DenseMap unit tests.

These were already trying to be type parameterized over different
key/value pairs. I've realized this goal using GoogleTest's typed test
functionality. This allows us to easily replicate the tests across
different key/value combinations and soon different mapping templates.

I've fixed a few bugs in the tests and extended them a bit in the
process as many tests were only applying to the int->int mapping.

llvm-svn: 158589