[lldb] Only set the executable module for a target once

Summary:

When we try to find the executable module for our target we don't check
if we already have an executable module set. This causes that when debugging
a program that dlopens another executable, LLDB will take that other executable
as the new executable of the target (which causes that future launches of the
target will launch the dlopen'd executable instead of the original executable).

This just adds a check that we only set the executable when we haven't already
found one.

Fixes rdar://63443099

Reviewers: jasonmolenda, jingham, teemperor

Reviewed By: jasonmolenda, teemperor

Subscribers: jingham, JDevlieghere

Differential Revision: https://reviews.llvm.org/D80724