Revert "[lldb] Use current execution context in SBDebugger"
This reverts commit 754ab803b8dc659e3645d369d1b5d6d2f97be29e.
As pointed out in https://reviews.llvm.org/D95761, this patch could lead to
having the wrong execution context in some situations (thanks Jim!).
D92164 is addressing the same issue and will replace this patch, so I'll
revert this one.