c++11 support: first work towards proper DUChain integration of lambda expressions

- a lambda expression now properly gets a function type associated
- the lambda declarator now is represented by a function context
which gets imported into the body of the lambda expression

known problems:
- the surrounding context is still implicitly imported into the
body context
- the return-type is not properly computed in cases where no
explicit trailing return type is given but only a single
return statement in the body instead
- lambda captures are still not handled at all