`A16-0-1`: Fix handling of `#else` and `#elif` in valid preprocessor conditionals

[lcartey]

Affected rules

• A16-0-1

Description

We erroneously identify otherwise acceptable conditionals with #else or #elif preprocessor directives as invalid, as the #else and #elif are not considered de facto valid.

This should be addressed by:

• Always considering #else acceptable, and relying on reporting any issues in the #..if directive.
• Considering #elif an acceptable directive within a preprocessor conditional, but still considering the #elif on its own merits.

Example

#ifdef MACRO_ENABLED
#include "test1.h"
#else
#include "test2.h"
#endif