When deserializing an Objective-C class, check whether we have another

When deserializing an Objective-C class, check whether we have another
declaration of that same class that either came from some other module
or occurred in the translation unit loading the module. In this case,
we need to merge the two redeclaration chains immediately so that all
such declarations have the same canonical declaration in the resulting
AST (even though they don't in the module files we've imported).

Focusing on Objective-C classes until I'm happy with the design, then
I'll both (1) extend this notion to other kinds of declarations, and
(2) optimize away this extra checking when we're not dealing with
modules. For now, doing this checking for PCH files/preambles gives us
better testing coverage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147123 91177308-0d34-0410-b5e6-96231b3b80d8