CodeGen, Itanium: Properly dllimport RTTI data

We would blindly assume that RTTI data should have the same linkage as
the vtable because we didn't think the RTTI data was external.  This
oversight stemmed because we didn't take dllimport into account.

This fixes PR21512.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221511 91177308-0d34-0410-b5e6-96231b3b80d8