Improve the Objective-C common-type computation used by the ternary operator.

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