[ADT] Add an early-increment iterator-like type and range adaptor.

This allows us to model the common LLVM idiom of incrementing
immediately after dereferencing so that we can remove or update the
entity w/o losing our ability to reach the "next".

However, these are not real or proper iterators. They are just enough to
allow range based for loops and very simple range algorithms to work,
but should not be considered full general.

Differential Revision: https://reviews.llvm.org/D49956

llvm-svn: 338955

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