[OPENMP 4.5] Codegen for member decls in 'lastprivate' clause.

OpenMP 4.5 allows to privatize non-static member decls in non-static
member functions. Patch captures such decls by reference in general (for
bitfields, by value) and then operates with this capture. For bitfields,
at the end of codegen for lastprivates original bitfield is updated with the value of captured copy.

llvm-svn: 261824