Exclude invalid old decl from mismatching linkage assertion

This patch fixes PR18964. In linkage computation, assertion fails when
an old invalid declaration's linkage mismatches with the current
decl's one.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203168 91177308-0d34-0410-b5e6-96231b3b80d8