Add the new Args / entry-access API.

The long-term goal here is to get rid of the functions
GetArgumentAtIndex() and GetQuoteCharAtIndex(), instead
replacing them with operator based access and range-based for
enumeration.  There are a lot of callsites, though, so the
changes will be done incrementally, starting with this one.

Differential Revision: https://reviews.llvm.org/D26883

llvm-svn: 287597