[lldb/API] Make Launch(Simple) use args and env from target properties

Summary:
When no arguments or environment is provided to SBTarget::LaunchSimple,
make it use the values surrently set in the target properties. You can
get the current behavior back by passing an empty array instead.

It seems like using the target defaults is a much more intuitive
behavior for those APIs. It's unllikely that anyone passed NULL/None to
this API after having set properties in order to explicitely ignore them.

One direct application of this change is within the testsuite. We have
plenty of tests calling LaunchSimple and passing None as environment.
If you passed --inferior-env to dotest.py to, for example, set
(DY)LD_LIBRARY_PATH, it wouldn't be taken into account.

Reviewers: jingham, labath, #libc_abi!

Subscribers: libcxx-commits, lldb-commits

Tags: #lldb, #libc_abi

Differential Revision: https://reviews.llvm.org/D76045