qa: Replace [[fallthrough]] by [[clang::fallthrough]]

qa: Replace [[fallthrough]] by [[clang::fallthrough]]

The [[fallthrough]] attribute is a C++17 extension. It happens to work
in GCC 7, but causes warnings or even failures in other compilers.

In my quest to find a syntax that makes all compilers happy without
having to disable the (useful) warning, I think I have finally
found a way. Using the clang:: namespace seems to solve several
issues:

* It is marked as a compiler extension, so compilers not supporting it
  should simply ignore the attribute
* It (obviously) works in Clang, which is nice when using a Clang-based
  IDE like Qt Creator
* For some reason, it also seems to silence warnings in GCC; I guess one
  of GCC's fancy regexes somehow matches