Sema: Only merge typedef attributes if the previous decl is a typedef

In r186373, we started merging attributes on typedefs, but this causes
us to try to merge attributes even if the previous declaration was not
a typedef.

Only merge the attributes if the previous decl was also a typedef.

Fixes rdar://problem/15044218

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192146 91177308-0d34-0410-b5e6-96231b3b80d8