Lambda closure types have a conversion function to a block pointer

Lambda closure types have a conversion function to a block pointer
with the same parameter types and return type as the function call
operator. This is the real answer to

  http://stackoverflow.com/questions/4148242/is-it-possible-to-convert-a-c0x-lambda-to-a-clang-block

:)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150620 91177308-0d34-0410-b5e6-96231b3b80d8