Emit the function type of member function pointer types the same as member functions.

Adding the pseudo first parameter to a member function pointer's function type
and mark it as artificial.

Combined with a fix to GDB ( http://sourceware.org/bugzilla/show_bug.cgi?id=14998 )
this fixes gdb.cp/member-ptr.exp with Clang.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172911 91177308-0d34-0410-b5e6-96231b3b80d8