[CommandLine] Fix cl::ConsumeAfter support with more than one positional argument

Summary:
Currently, cl::ConsumeAfter only works for the case that has exactly one
positional argument. Without the fix, it skip fulfilling first positional
argument and put that additional positional argument in interpreter arguments.

Reviewers: bkramer, Mordante, rnk, lattner, beanz, craig.topper

Reviewed By: rnk

Subscribers: JosephTremoulet, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77242