When deserializing the definition of a C++ class/ObjC class/ObjC

When deserializing the definition of a C++ class/ObjC class/ObjC
protocol, record the definition pointer in the canonical declaration
for that entity, and then propagate that definition pointer from the
canonical declaration to all other deserialized declarations. This
approach works well even when deserializing declarations that didn't
know about the original definition, which can occur with modules.

A nice bonus from this definition-deserialization approach is that we
no longer need update records when a definition is added, because the
redeclaration chains ensure that the if any declaration is loaded, the
definition will also get loaded.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148223 91177308-0d34-0410-b5e6-96231b3b80d8