[PP] Replace some uses of unsigned with size_t

All values are returned by a method as size_t, and subsequently passed
to functions taking a size_t, or used where a size_t is also valid.
Better still, two loops (which had an unsigned), can be replaced by
a range-based for loop.

Differential Revision: http://reviews.llvm.org/D25939

llvm-svn: 285182