Bug 872644 - Rename UnsafeSetElement intrinsic to UnsafePutElements. r=shu

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extra : rebase_source : 0f7e49b1a29a33b38b6be3c77073e11d3d6373c9
extra : amend_source : 529aa068a6592e8f1b1c818dc7d29612b3528908