Fix nested type names for properties

[dpaoliello]

When ClangSharp generates the type name for a property, if the type of the property is a nested type, then it walks from the parent of the current type to the root type and appends the names as it goes. This means that if the nesting is more than one type deep, then the types will be backwards.

For example, given:

typedef union _MY_UNION
{
    long AsArray[2];
    struct {
        long First;
        union {
            struct { long Second; } A;
            struct { long Second; } B;
        }};
    }};
}} MY_UNION;

ClangSharp generates:

public ref _Anonymous_e__Union._Anonymous_e__Struct._B_e__Struct B {
    get {
        fixed (_Anonymous_e__Struct._Anonymous_e__Union* pField = &Anonymous.Anonymous) {
            return ref pField->B;
        }
    }
}

So the type used when pinning the field is correct, but the type for the property itself is incorrect.

The fix is to prepend each type during the walk, then prepend the ref keyword as required.

I also switched the code to use a StringBuilder.