[libc] Add a testing macro for MPFR matchers skipping `explainError` calls.

Adding `EXPECT_MPFR_MATCH_ROUNDING_SILENTLY` macro that does not call
`explainError` when the tests fail.  This is useful to check the passing or
failing rates, such as hitting percentages of fast passes in math
implementations.

Reviewed By: michaelrj, sivachandra

Differential Revision: https://reviews.llvm.org/D136731