Level 2 warnings are considered "significant" and turning them on
revealed one issue that could, in the future, have affected MSVC-ABI
builds using either CL or Clang.
MSVC's C++ name mangling distinguishes between classes and structs, and
throughout the ares codebase there were friend declarations using
"class" to refer to types that were defined with "struct".
I don't see any value in retaining this inconsistency, especially
considering that ares otherwise avoids the "class" keyword (though I
spotted a few uses that crept in from recent commits).
