[IR] Add NODISCARD to attribute functions
Summary: Many functions on `llvm::AttributeList` and `llvm::AttributeSet` are documented with "returns a new {list,set} because attribute {lists,sets} are immutable." This documentation can be aided by the addition of an attribute, `LLVM_NODISCARD`. Adding this prevents unsuspecting users of the API from expecting `AttributeList::setAttributes` from modifying the underlying list. At the very least, it would have saved me a few hours of debugging, since I had been doing just that! I had a bug in my program where I was calling `setAttributes` but then passing in the unmutated `AttributeList`. I tried adding LLVM_NODISCARD and confirmed that it would have made my bug immediately obvious. Reviewers: rnk, javed.absar Reviewed By: rnk Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D55217 llvm-svn: 348372
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