This warning very rarely useful (it's a hint and not mandatory) and it
causes many warnings to be printed when compiling C++ code that uses the
standard library with GCC.
This flag was originally added in back in r94332 but the flag is a no-op
in Clang ("This diagnostic flag exists for GCC compatibility, and has no
effect in Clang"). Removing it should make the GCC build output slightly
more readable.