Check for internal weak decls after merging.

This fixes pr14946. The problem was that the linkage computation was done too
early, so things like "extern int a;" would be given external linkage, even if
a previous declaration was static.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172667 91177308-0d34-0410-b5e6-96231b3b80d8