Merging r353701:

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r353701 | mgorny | 2019-02-11 06:09:48 -0800 (Mon, 11 Feb 2019) | 8 lines

[lldb] [lit] Fix finding lld-link when it is not in 'compiler dir'

Fix the build helper to find lld-link via PATH lookup, rather than
making a fragile assumption that it will be present in the 'compiler
directory'.  This fixes tests on Gentoo where clang and lld
are installed in different directories.

Differential Revision: https://reviews.llvm.org/D58001
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llvm-svn: 362017