Allow creating fallthrough macros

[DanielGibson]

Is your feature request related to a problem? Please describe.

It's useful to have a fallthrough macro that can be used in switch-case blocks, to indicate that falling through is intended.

The CDT parser already supports using a comment for this, like // fallthrough, but that doesn't help if your project (like the Linux kernel source) already uses a macro for this.

In their case resolving to __attribute__((__fallthrough__)), which CDT doesn't support either (see also this older bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=567227 ).
For C++ [[fallthrough]] is supported, but that doesn't help for C.

Describe the solution you'd like

Currently Linux has this code:

#if __has_attribute(__fallthrough__)
# define fallthrough                    __attribute__((__fallthrough__))
#else
# define fallthrough                    do {} while (0)  /* fallthrough */
#endif

in a header.

Used like

	switch (*data) {
	default:
		// ... some code ...
		fallthrough;	/* report timeout */
	case 0xfc:
		// ... some more code ...

I could (locally) adjust it to

#ifdef __CDT_PARSER__
# define fallthrough                    __CDT_FALLTHROUGH__
#elif __has_attribute(__fallthrough__)
// ...

.. if __CDT_FALLTHROUGH__ (or something similar) existed.

Describe alternatives you've considered

Maybe just supporting __attribute__((__fallthrough__)) directly would be even better. Or both?