Clang tidy: adaptions for v5 and fixes
This MR addresses
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make .clang-tidy an actual build dependency
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deal with newly introduced checks in clang-tidy v5
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fixes for several clang-tidy findings
Most of them were probably hidden by a Linux-only clang-tidy run. Running clang-tidy on OSX revealed some leftovers that could mostly be fixed by clang-tidy itself (-fix)