To prevent crashes during `exit` caused by SR-12668, avoid running
static destructors. During testing, we do this by interposing
`__cxa_exit` to prevent registration of destructors. This depends on the
previous commit that disabled RTLD_DEEPBIND. In the sourcekit-lsp
executable, since we control the exit points, we can make a less
invasive change to just use `_Exit` instead of `exit` to avoid cleanups.
This should hopefully fix the CI issues we have seen when testing with
--parallel.
rdar://61837752
