Implicitly define a lambda's conversion functions (to function

pointers and block pointers). We use dummy definitions to keep the
invariant that an implicit, used definition has a body; IR generation
will substitute the actual contents, since they can't be represented
as C++. 

For the block pointer case, compute the copy-initialization needed to
capture the lambda object in the block, which IR generation will need
later.

llvm-svn: 150645