wcc: c-side mangles private decls; main exempt as entry-point convention

cmd/wcc/ww.h

@@ -196,6 +196,7 @@ struct Lex {
 	i32	col;
 	Arena	*a;		/* token-text arena */
 	int	errs;
+	const char *module;	/* current `// MODULE: foo` directive, or NULL */
 };
 
 void	lexinit(Lex*, Arena*, const char *file, const char *src, u64 len);
@@ -303,6 +304,11 @@ struct Node {
 	int		export;
 	Type		*type;	/* filled in by checker */
 	const char	*tsuffix;	/* typed numeric literal suffix */
+	const char	*module;	/* `// MODULE: foo` directive at the
+					 * decl's section in combined.ww.
+					 * NULL for nested nodes; only top-
+					 * level decls (fn/def/type/let)
+					 * carry it. */
 };
 
 Node	*newnode(Arena*, Nkind, Pos);