TryConsume parser cleanups

Also move some comments into the block they were meant to describe.
Update LangOpt descriptions

Based on recent discussions, attempt to provide a clearer distinction between
MicrosoftMode and MicrosoftExt. This still doesn't feel perfect but gives a
better idea which is which.

Also update the CPlusPlus11 description which got missed in r171367.

C++0x is dead, long live C++0x!