Support statement attributes

[lambdageek]

GCC Attribute Syntax allows the following:

In GNU C, an attribute specifier list may appear as part of a null statement. The attribute goes before the semicolon.

ie, this is currently a syntax error in language-c:

switch (e) {
  case 1:
         foo ();
         __attribute__((fallthrough)) ;
   default:
         bar ();
}

Unfortunately adding the naive additional production to statement leads to two extra shift/reduce conflicts and it's not obvious to me if they're benign or problematic.

statement ::
  ...
  | attrs ';'                   {% withNodeInfo $1 $ CAttrStmt $1 }