gcc version for the lldb buildbot (gcc20)

Hi Duncan,

Some recent commits broke the LLDB buildbot due to use of:

- nullptr
- delegating constructors

In the past, we've changed c++11 unsupported features back to their c++03 equivalents, but since this feels like "going backwards", and also because I'm relishing the opportunity to use more C++11 goodness, I'm wondering if it would be sensible and possible to update the gcc20 builder? We have reports on IRC that GCC 4.7 is able to build LLDB ok.