Before this change StringName used regular static field
definitions for its mutex, _table, configured and debug_stringname
fields.
Since in the general case the ordering of the static variable and field
initialization and destruction is undefined, it was possible that
the destruction of StringName's static fields happened prior to
the destruction of statically allocated StringName instances.
By changing the static field definitions to inline in string_name.h,
the C++17 standard guarantees the correct initialization and destruction
ordering.
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