[lldb] Duplicate Target::Launch resuming logic into CommandObjectPlatformProcessLaunch

Fix `platform process launch` on macOS where it fails for lack of auto-resuming support.

This change reproduces the resuming logic found in `Target::Launch`.

This issue was identified by @DavidSpickett in D153636. This change relies on the tests
added in that PR. Thanks David.

Differential Revision: https://reviews.llvm.org/D153922