Implementation of nested loops in cxx_loop_proto

Summary: Extended `cxx_loop_proto` to have neste for loops. Modified `loop_proto_to_llvm` and `loop_proto_to_cxx` to handle the new protos. All protos have a set of statements designated as "inner loop" statements and a set of statements designated as "outer loop" statements.

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D50670

llvm-svn: 339832