[libc] Improve information printed on failure of a math test which uses MPFR.

A new test matcher class MPFRMatcher is added along with helper macros
EXPECT|ASSERT_MPFR_MATCH.

New type traits classes RemoveCV and IsFloatingPointType have been
added and used to implement the above class and its helpers.

Reviewers: abrachet, phosek

Differential Revision: https://reviews.llvm.org/D79256