Basic support for name mangling of C++11 lambda expressions. Because

name mangling in the Itanium C++ ABI for lambda expressions is so
dependent on context, we encode the number used to encode each lambda
as part of the lambda closure type, and maintain this value within
Sema.

Note that there are a several pieces still missing:
  - We still get the linkage of lambda expressions wrong
  - We aren't properly numbering or mangling lambda expressions that
  occur in default function arguments or in data member initializers.
  - We aren't (de-)serializing the lambda numbering tables

llvm-svn: 150982