[PATCH] D26568: [ADT] Delete Twine constructors which accept StringRef rvalues (NFC)

[Duncan P. N. Exon Smith via llvm-commits]

Deleting operator= SGTM too.

> On 2016-Nov-14, at 11:15, Jordan Rose <jordan_rose at apple.com> wrote:
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> jordan_rose added a comment.
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> I'm inclined to agree with @dblaikie's email comment: assigning a Twine is pretty much never safe anyway, so we should just delete `operator=` altogether. (I'd love to get rid of the copy-constructor too and just allow moves, but I think it still makes sense to copy-construct when combining a Twine argument with multiple different suffixes.)
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> https://reviews.llvm.org/D26568
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