Merging r366573:

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r366573 | nico | 2019-07-19 15:29:10 +0200 (Fri, 19 Jul 2019) | 6 lines

lld-link: Demangle symbols from archives in diagnostics

Also add test coverage for thin archives (which are the only way I could
come up with to test at least some of the diagnostic changes).

Differential Revision: https://reviews.llvm.org/D64927
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llvm-svn: 369881