[ADT] llvm::bsearch, binary search for mere mortals

Summary:
Add to STLExtras a binary search function with a simple mental model:
You provide a range and a predicate which is true above a certain point.
bsearch() tells you that point.
Overloads are provided for integers, iterators, and containers.

This is more suitable than std:: alternatives in many cases:
 - std::binary_search only indicates presence/absence
 - upper_bound/lower_bound give you the opportunity to pick the wrong one
 - all of the options have confusing names and definitions when your predicate
   doesn't have simple "less than" semantics
 - all of the options require iterators
 - we plumb around a useless `value` parameter that should be a lambda capture

The API is inspired by Go's standard library, but we add an extra parameter as
well as some overloads and templates to show how clever C++ is.

Reviewers: ilya-biryukov, gribozavr

Subscribers: dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60779

llvm-svn: 358540