Catch2's documentation promises that listeners are called _before_
reporters, but because of the previous implementation, they were
called _after_ reporters. This commit fixes that.
Closes#1234
An empty default implementation is provided to keep backward compatibility.
Called when signal or Structured Exception is encountered.
Related to #1005
Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)
The last three are enabled for Clang compilation going forward.
Also enabled `-Wunreachable-code` for Clang and GCC
