[ADT] Move drop_begin from iterator_range.h into STLExtras.

Summary:
drop_begin depends on adl_begin/adl_end, which are defined in STLExtras.h,
but we can't just #include STLExtras.h in iterator_range.h as that would
introduce a circular reference (STLExtras.h already depends on
iterator_range.h). The simplest solution is to move drop_begin into
STLExtras.h, which is a reasonable home for it anyway.

Reviewers: dblaikie

Subscribers: dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70189