The Objective-C common-type computation had a few problems that
required a significant rework, including:
- Quadradic behavior when finding the common base type; now it's
linear.
- Keeping around type arguments when computing the common type
between a specialized and an unspecialized type
- Introducing redundant protocol qualifiers.
Part of rdar://problem/6294649. Also fixes rdar://problem/19572837 by
addressing a longstanding bug in
ASTContext::CollectInheritedProtocols().
llvm-svn: 241544