Convert all python objects in our API to use overload the __str__ method

instead of the __repr__. __repr__ is a function that should return an
expression that can be used to recreate an python object and we were using
it to just return a human readable string.

Fixed a crasher when using the new implementation of SBValue::Cast(SBType).

Thread hardened lldb::SBValue and lldb::SBWatchpoint and did other general
improvements to the API.

Fixed a crasher in lldb::SBValue::GetChildMemberWithName() where we didn't
correctly handle not having a target.

llvm-svn: 149743