Introduce lldb-env, a tool that prints out the environment variables
needed to launch lldb.
Usually, no environment variables need to be set to launch lldb: in
these cases lldb-env prints nothing.
In a sanitized build, lldb-env prints out any mandatory DYLD_* options
needed to set up sanitizer interceptors early. The Darwin-specific logic
is just a starting point. The end goal would be to support launching
lldb on any platform via export $(path/to/lldb-env); path/to/lldb. The
current logic is taken/moved from test/API/lit.cfg. As a follow-up, we
could have lldb-dotest use lldb-env as well.
Testing: check-lldb with LLVM_USE_SANITIZER='Address'