It is provided by `functional`, not `utility`.  Fix the mapping added by
commit 276d3c7a (IWYU: Add workaround mapping for std::hash,
2018-07-31).  Also generalize the workaround from commit v3.12.0-rc1~39^2~1
(IWYU: Define a macro to tell code it is preprocessing for iwyu,
2018-05-25) to allow local builds to configure specific flags.  This
is needed because iwyu behaves differently in different environments.