A few extras were fixed

- Symbol::GetAddress() now returns an Address object, not a reference. There were places where people were accessing the address of a symbol when the symbol's value wasn't an address symbol. On MacOSX, undefined symbols have a value zero and some places where using the symbol's address and getting an absolute address of zero (since an Address object with no section and an m_offset whose value isn't LLDB_INVALID_ADDRESS is considered an absolute address). So fixing this required some changes to make sure people were getting what they expected. 
- Since some places want to access the address as a reference, I added a few new functions to symbol:
    Address &Symbol::GetAddressRef();
    const Address &Symbol::GetAddressRef() const;

Linux test suite passes just fine now.

<rdar://problem/21494354>

llvm-svn: 240702