Extract the instance-method case for debug info out into a separate function.

This is in preparation for using this to construct the function type for
pointers to member functions to include the implicit/artificial 'this'
parameter in that case as well. (feedback from GDB indicates that this might be
all that's necessary to get it to behave well with Clang's pointer-to-member
function debug output)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171809 91177308-0d34-0410-b5e6-96231b3b80d8