When importing classes and structs with anonymous structs, it is critical that

When importing classes and structs with anonymous structs, it is critical that
distinct anonymous structs remain distinct despite having similar layout.

This is already ensured by distinguishing based on their placement in the parent
struct, using the function `findAnonymousStructOrUnionIndex`.

The problem is that this function only handles anonymous structs, like
```
class Foo { struct { int a; } }
```
and not untagged structs like
```
class Foo { struct { int a; } var; }
```
Both need to be handled, and this patch fixes that.  The test case ensures that this functionality doesn't regress.

Thanks to Manman Ren for review.

https://reviews.llvm.org/D22270

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275460 91177308-0d34-0410-b5e6-96231b3b80d8