Merging r373049:

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r373049 | hans | 2019-09-27 01:14:45 -0700 (Fri, 27 Sep 2019) | 4 lines

Fix the 'directory' field in DumpCompilationDatabase and add test

This broke in r371027 due to a missing negation
(llvm::sys::fs::current_path returns false on success).
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