[lldb] Fix assertion when ScriptedProcess have no pid after launch

This patch should fix an assertion that causes some test failures:
https://ci.swift.org/view/LLDB/job/llvm-org-lldb-release-debuginfo/3587/console

This was caused by the changes introduces in `88f40919` where we
replaced `DidLaunch` by `DidResume` in the `ScriptedProcess` class.

However, by the time we resume the process, the pid should be already
set. To address this, this patch brings back `DidLaunch` which will
initialize the ScriptedProcess pid with a placeholder value. That value
will be updated in `DidResume` to the final pid.

Note, this 2 stage PID initialization is necessary sometimes, when the
scripted process gets stopped at entry (launch) and gets assigned an
object that contains the PID value. In this case, we need to update the
PID when we resume the process after we've stopped at entry.

This also replaces the default scripted process id to an arbitrary
number (42) since the current value (0) is considered invalid.

Differential Revision: https://reviews.llvm.org/D148153



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>