qa: Add [[fallthrough]] annotations where appropriate

qa: Add [[fallthrough]] annotations where appropriate

Newer GCCs start to warn about this, so add attributes or rearrange
code as appropriate.

While the [[fallthrough]] attribute has only been standardized in
C++17, it should simply be ignored by older compilers (and it does
shut up GCC even when compiling with -std=c++11).

(cherry picked from commit 45affd4fa815bb21d0b2e46ac80114bb9174f801)