[lldb] Call os_log_fault on lldb_assert

Call `os_log_fault` when an lldb assert fails. We piggyback off
`LLVM_SUPPORT_XCODE_SIGNPOSTS`, which also depends on `os_log`, to avoid
having to introduce another CMake check and corresponding define.

This patch also adds a small test using lldb-test that verifies we abort
with a "regular" assertion when asserts are enabled.

Differential revision: https://reviews.llvm.org/D98987