[unittest] teach gTest to print entries of DenseMap as pairs

When an assertion like the following fails:
   EXPECT_THAT(map, ElementsAre(Pair("p", "nullable"))));
Error message before:
   Actual: { 40-byte object <E8-A5 9C-7F 25-37 00-00 58-7E 51-51 D0-7F 00-00 00-00 00-00 00-00 00-00 01-00 00-00 00-00 00-00 00-DA C7-7F 25-37 00-00> }
After:
   Actual: { ("p", "nonnull") }

It is not ideal that we need to refer directly to DenseMapPair inside the
internal namespace, but I believe the practical maintenance risk is low.
This change is covered by DenseMap's unittests, as we've covered SmallString etc
in the past.

Differential Revision: https://reviews.llvm.org/D153930