[libc] Fix libmath test compilation when using UInt<T>

This patch:
(1) adds the add_with_carry_const and sub_with_borrow_const constexpr calls
to add and sub, respectively. Both add and sub are constexpr calls and
were call the non-constexpr version of add/sub_with_borrow.
(2) adds explicit UIntType construct calls in some fp tests.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D150223