Merge handleDLLImportAttr and handleDLLExportAttr into one function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211647 91177308-0d34-0410-b5e6-96231b3b80d8
MS ABI: Ignore dll attributes on partial template specializations

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211648 91177308-0d34-0410-b5e6-96231b3b80d8