Now that deserializing a definition of a C++ class/Objective-C

Now that deserializing a definition of a C++ class/Objective-C
class/Objective-C protocol suffices get all of the redeclarations of
that declaration wired to the definition, we no longer need to record
the identity of the definition in every declaration. Instead, just
record a bit to indicate whether a particular declaration is the
definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148224 91177308-0d34-0410-b5e6-96231b3b80d8