[lldb] Fix compilation with gcc-6.5
This fixes (works around) two errors with gcc-6.5.
- in the RegisterContext_x86 files, gcc is unable to synthesize a default constructor -- it thinks it needs to initialize the virtual base class, even though said classes are abstract. I fix that by providing a dummy constructor.
- In ReproducerInstrumentationTest, it is not able to deduce that the TestingRegistry class is movable (it contains a map of unique pointers). I change the type from Optional<TestingRegistry> to unique_ptr<TestingRegistry), so that moving is not required (copying/moving a polymorphic type is not a very good idea in any case).