[AST] Revert mangling changes from r339428

As discussed in https://reviews.llvm.org/D50144, we want Obj-C classes
to have the same mangling as C++ structs, to support headers like the
following:

```
@class I;
struct I;

void f(I *);
```

since the header can be used from both C++ and Obj-C++ TUs, and we want
a consistent mangling across the two to prevent link errors. Itanium
mangles both the same way, and so should the MS ABI.

The main concern with having the same mangling for C++ structs and Obj-C
classes was that we want to treat them differently for the purposes of
exception handling, e.g. we don't want a C++ catch statement for a
struct to be able to catch an Obj-C class with the same name as the
struct. We can accomplish this by mangling Obj-C class names differently
in their RTTI, which I'll do in a follow-up patch.

Differential Revision: https://reviews.llvm.org/D52581

llvm-svn: 343808