To be used around code where some compiler, in some circumstances,
generates bogus warnings about unreachable code, that it would be much
uglier to work around otherwise.

Specifically, I will at first now use this to get rid of MSVC warnings
about unreachable code when calling a function defined in another
source file (but going into the same library) that always throws. The
compiler notices this when one uses link-time code generation and it
thus can do global inlining of code from all compilation units that go
into a library (or executable).

For MSVC, the __pragma that the SAL_WNOUNREACHABLE_CODE_PUSH macro
expands to needs to be in force at the start curly brace of a
function, so place the PUSH macro before the function definition. For
clarity, I guess it is best that the corresponding POP macro comes
after the end of the function.

Change-Id: Icef5259c5360b9facdc136fec1f207665ce79d90