Add empty stub for a LLDB injector.

Introduce a common base class with the GDB injector, they will share the
basic logic, just the specific interaction with the debugger is different.
Move process handling and self-test to the base class.
Refactor launch() and attach() to be shared between GDB and LLDB.
Refactor symbol loading commands.

They don't exist with LLDB at all. Also, fix QtCore loading for Qt5.
Remove a bizarre and no longer needed workaround.

This breaks with LLDB. Also, clean up the questionable string handling
here to avoid usage of temporaries.
Fix LLDB runtime attaching.
Add stdout reading for LLDB for debugging as well.

We need to do this here to not interfere with the waiting for the command
prompt.
Detect automatically whether to use gdb or lldb.