[ADT] drop_begin: use adl_begin/adl_end. NFC.

Summary:
The instantiation of the drop_begin function template usually fails because the functions begin() and end() do not exist. Only when using on a container from the std namespace (or `llvm::iterator_range`s of something derived from `std::iterator`), they are matched to std::begin() and std::end() due to Koenig-lookup.

Explicitly use llvm::adl_begin and llvm::adl_end to make drop_begin applicable to anything iterable (including C-style arrays).

A solution for general `llvm::iterator_range`s was already tried in r244620, but got reverted in r244621 due to MSVC not liking it.

Reviewers: dblaikie, grosbach, aaron.ballman, ruiu

Reviewed By: dblaikie, aaron.ballman

Subscribers: aaron.ballman, llvm-commits

Differential Revision: https://reviews.llvm.org/D48598

llvm-svn: 335772