[ADT] Warn on unused results from ArrayRef and StringRef functions that read...

[ADT] Warn on unused results from ArrayRef and StringRef functions that read like they might mutate.

Summary:
Functions like "slice" and "drop_front" sound like they might mutate the
underlying object, but they don't.  Warning on unused results would have
saved me an hour yesterday, and I'm sure I'm not the only one.

LLVM and Clang are clean wrt this warning after D22540.

Reviewers: majnemer

Subscribers: sanjoy, chandlerc, llvm-commits

Differential Revision: https://reviews.llvm.org/D22541

llvm-svn: 276058