AST: support protocol conformances on id/class/interfaces in MS ABI

Add support for mangling ObjC protocol conformances in MS ABI as if they are
COM interfaces. By diverging from the itanium mangling of `objc_protocol`
prefixed names, this approach allows for a semi-reasonable, albeit of
questionable sanity, undecoration via existing tooling. There is also the
possibility of adding an extension and taking part of the namespace to add the
conformance via the `L` and `Z` "modifiers", but the existing tooling would not
be able to properly undecorated the symbol even though incidentally `undname`
currently produces something legible while wine's implementation is not able to
cope with the extension.

This allows for the disambiguation of overloads where the parameter differs
only in the protocol conformance of the ObjC type, e.g.

```
@protocol P;
void f(std::vector<id>);
void f(std::vector<id<P>>);
```

which clang would previously fail due to the mangling being identical as the
protocol conformance was ignored.

llvm-svn: 323547