Move the SmallVector unit tests to be type-parameterized so that we can

test more than a single instantiation of SmallVector.

Add testing for 0, 1, 2, and 4 element sized "small" buffers. These
appear to be essentially untested in the unit tests until now.

Fix several tests to be robust in the face of a '0' small buffer. As
a consequence of this size buffer, the growth patterns are actually
observable in the test -- yes this means that many tests never caused
a grow to occur before. For some tests I've merely added a reserve call
to normalize behavior. For others, the growth is actually interesting,
and so I captured the fact that growth would occur and adjusted the
assertions to not assume how rapidly growth occured.

Also update the specialization for a '0' small buffer length to have all
the same interface points as the normal small vector.

llvm-svn: 161001