[lib] Make lib compatible with -Wfall-through excepting legacy

[terrelln]

Switch to a macro ZSTD_FALLTHROUGH; instead of a comment. On supported
compilers this uses an attribute, otherwise it becomes a comment.

This is necessary to be compatible with clang's -Wfall-through, and
gcc's -Wfall-through=2 which don't support comments. Without this the
linux build emits a bunch of warnings.

Also add a test to CI to ensure that we don't regress.

[Cyan4973]

This feature was also recently added to xxhash,
and I remember it wasn't completely trivial to ensure it would work correctly with a broad range of compilers.
The macro initialization ended up being a bit complex :
https://github.com/Cyan4973/xxHash/blob/dev/xxhash.h#L598
That being said, it works now. Might be useful for inspiration.

[terrelln]

It looks like this section can be simplified with __has_attribute(__fallthrough__) as I'm doing in zstd. Clang always supported it, and gcc started supporting __has_attribute() before it supported __fallthrough__.

We can definitely add on the section that uses the C/C++ language attribute [[fallthrough]] though.

[Cyan4973]

Apparently, the macro determination code in xxhash uses __has_c_attribute(), instead of __has_attribute().
Maybe because it's looking for fallthrough, instead of __fallthrough__.

[terrelln]

Apparently, the macro determination code in xxhash uses __has_c_attribute(), instead of __has_attribute().

__has_c_attribute() is to detect support for [[fallthrough]], where __has_attribute() detects support for __attribute__((__fallthrough__)).

[Cyan4973]

I presume this will not add a comment where this macro is invoked ?