Change hasUniqueInitializer() to call isStrongDefinitionForLinker() instead of !isWeakForLinker()

Summary:
Available_externally global variable with initializer were considered "hasInitializer()",
while obviously it can't match the description:

    Whether the global variable has an initializer, and any changes made to the
    initializer will turn up in the final executable.

since modifying the initializer of an externally available variable does not make sense.

Reviewers: pcc, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15351

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 255123