Merging r280599:

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r280599 | mehdi.amini | 2016-09-03 14:12:33 -0700 (Sat, 03 Sep 2016) | 10 lines

Fix ThinLTO crash with debug info

Because the recent change about ODR type uniquing in the context,
we can reach types defined in another module during IR linking.
This triggered some assertions in case we IR link without starting
from an empty module. To alleviate that, we can self-map metadata
defined in the destination module so that they won't be visited.

Differential Revision: https://reviews.llvm.org/D23841

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llvm-svn: 287137