Invalid rewriting of multi-decl containing forward-declared struct

[mattmccutchen-cci]

I was kicking the tires of 3C's rewriting of multi-decls containing structs a bit in preparation for working on #531, and I discovered that the current code doesn't handle forward declarations of structs properly. The following file:

struct fwd *p;

rewrites to the following syntatically invalid code, with or without -alltypes:

struct }_Ptr<struct fwd> p = ((void *)0);

My understanding of Clang's AST parsing behavior is that the first mention of struct fwd in any context in a translation unit is reported as a RecordDecl whether or not it has a body. If it has a body, it is the definition, otherwise it is a forward declaration; we can use RecordDecl::isCompleteDefinition to distinguish between these cases. Subsequent occurrences of struct fwd; by itself are reported as forward declarations, but subsequent mentions of struct fwd as part of other constructs are not reported as RecordDecls at all.

AFAIK, "forward declarations" of structs as part of other constructs have no significance in C, because the first mention of a struct, wherever it appears, will be treated as a forward declaration. (There is one edge case in which a standalone forward declaration does make a difference, but 3C won't modify those anyway.) So probably what we want to do is have all 3C code that handles struct definitions in multi-decls just ignore RecordDecls for which isCompleteDefinition returns false. I prototyped this change and it seemed to have the desired effect in the example above; I'll probably submit a PR later.