[libcxx] Add flag to disable __builtin_assume in _LIBCPP_ASSERT

Introduce _LIBCPP_ASSERTIONS_DISABLE_ASSUME which makes _LIBCPP_ASSERT
not call __builtin_assume when _LIBCPP_ENABLE_ASSERTIONS == 0.

Calling __builtin_assume was introduced in D122397.

__builtin_assume is generally supposed to improve optimizations, but can
cause regressions when LLVM has trouble handling the calls to
`llvm.assume()` (see comments in D122397).

Reviewed By: philnik

Differential Revision: https://reviews.llvm.org/D123175