[ADT] Implement llvm::bsearch() with std::partition_point()

Summary:
Delete the begin-end form because the standard std::partition_point
can be easily used as a replacement.

The ranges-style llvm::bsearch will be renamed to llvm::partition_point
in the next clean-up patch.

The name "bsearch" doesn't meet people's expectation because in C:

> If two or more members compare equal, which member is returned is unspecified.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D63718

llvm-svn: 364719