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ARROW-4009: [CI] Run Valgrind and C++ code coverage in different builds

Also fix a couple ASAN / UBSAN issues (not all of them).

Author: Antoine Pitrou <antoine@python.org>

Closes #3302 from pitrou/ARROW-4009-separate-codecov-valgrind and squashes the following commits:

7dacf9e1 <Antoine Pitrou> Mark function inline
06372930 <Antoine Pitrou> Use clang on non-coverage C++ job
eca54b69 <Antoine Pitrou> ARROW-4009:  Run Valgrind and C++ code coverage in different builds