[ADT] Move DenseMapInfo for ArrayRef/StringRef into respective headers (NFC)

This is a followup to D103422. The DenseMapInfo implementations for
ArrayRef and StringRef are moved into the ArrayRef.h and StringRef.h
headers, which means that these two headers no longer need to be
included by DenseMapInfo.h.

This required adding a few additional includes, as many files were
relying on various things pulled in by ArrayRef.h.

Differential Revision: https://reviews.llvm.org/D103491