Incomplete struct types

[nurmukhametov]

The following examples are crashed:

struct S1 { struct S2 a; };
struct S1 func() {
    struct S1 x;
    return x;
}

struct Sx bar() {}

int boo(struct Sy) {}

int foo() { struct Sx x; }

I guess the reason for all of them is same. By some reason, ISPC doesn't check the usage of incomplete structs types on AST level. So, LLVM IR is incorrect:

Cannot allocate unsized type
  %7 = alloca %v4_varying_Sx, align 8
LLVM ERROR: Broken module found, compilation aborted!

This example is compiled but shouldn't. It has to generate the compiler error "error: field 'a' has incomplete type 'S2'"

struct S1 { struct S2 a; };

We also want to generate warning like this "Error: Enum type "Ab" unknown. Did you mean "Aa"" when user misspels struct type name.