Add macro LLVM_ATTRIBUTE_REINITIALIZES

Summary:
This marks legitimate use-after-move (e.g. `Found.clear()` in rC342925)
which would otherwise be caught by bugprone-use-after-move.

bugprone-use-after-move recognizes this attribute after rCTE339571.

Reviewers: aaron.ballman, rsmith, mboehme, hokein

Reviewed By: mboehme

Subscribers: kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D52451

llvm-svn: 342949