C grammar should capture acceptable but non-standard C code

[bdamm]

Production C code may be extended with non-standard grammars. For example:

• "__attribute__((packed))" and "__attribute((suppress))" may be after parameters, type qualifiers, function declarations, and even anonymous unions. It's tempting to consider it a sub to Spacing given how widely declared it can be.
• An empty file with nothing but comments is considered valid by at least clang and gcc. The grammar rejects it for requiring at least one ExternalDeclaration.
• gcc (did not test clang) will accept a function declaration terminated with a SEMI:

int func() { /* code in here. Or not. */ }; // <-- Note the SEMI.

• Also, a struct with an empty declarator is considered valid:

struct a { int b;  ;  };

• "__inline" appears to be an acceptable alternative to "inline" for clang. Didn't test gcc on that one.
• An "asm" keyword and a non-trivial grammar is allowed by gcc and clang (and they might not be quite the same grammar, not 100% sure yet.)
• "\%" is parsed by gcc and clang as a valid escape (and is reduced to just %) but the grammar rejects this.
• __attribute__((format(printf, blah, etc))) also see, but disallowed by the current grammar.

Once I figure out pull requests I can get provide a diff with all of these.

[bdamm]

I'm going to go ahead and prepare a pull request for these changes. They should help the grammar be real-world useful, even though it won't any longer be strictly standards compliant.

[bdamm]

I've done what I can, I decided not to address __attribute in the grammar and instead stripped it out in preprocessing. So no more work on this from me.