Merging r354029:

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r354029 | mgorny | 2019-02-14 14:52:31 +0100 (Thu, 14 Feb 2019) | 7 lines

[lldb] [MainLoop] Report errno for failed kevent()

Modify the kevent() error reporting to use errno rather than returning
the return value.  At least on FreeBSD and NetBSD, kevent() always
returns -1 in case of error, and the actual error is returned via errno.

Differential Revision: https://reviews.llvm.org/D58229
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llvm-svn: 354254