Add MOZ_FALLTHROUGH macro definition for gcc 7 to suppress -Wimplicit-fallthrough warnings

The generic fallback MOZ_FALLTHROUGH definition is insufficient for GCC 7 and above, resulting in build warning spam and --enable-warnings-as-errors builds failing. The check for clang support is changed to use the __has_cpp_attribute macro, which is more robust than checking the __cplusplus version. Also, MOZ_FALLTHROUGH is now only defined in C++ code, since GCC errors out if it encounters a scoped attribute being used with __has_cpp_attribute in C code. No C code uses MOZ_FALLTHROUGH or derivatives at the moment.

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