Bug 1355692 - Add MOZ_FALLTHROUGH macro definition for gcc 7 to suppress -Wimplicit-fallthrough warnings. r=glandium

The generic fallback MOZ_FALLTHROUGH definition is insufficient for GCC 7 and
above, resulting in --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.

MozReview-Commit-ID: 4nKFBRD5jSF

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extra : rebase_source : 0c37ae39c806ca24a3271d3ec19531dd16e05daf