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