[libcxx] Readjust nodiscard_extensions.pass.cpp test - just disable for ASAN

In rL342814, i have committed a blind fix to unbreak the asan buildbot,
but as it was later discussed, the leak is intentional,
so we can not fix the failure that way.

So this reverts the leak 'fix',
and simply disables the test in the presence of ASAN.

llvm-svn: 342819