Common functionality is already checked within SemaDeclAttr.cpp and so it does not need to be re-checked for each target.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196428 91177308-0d34-0410-b5e6-96231b3b80d8
Giving a Subjects list to DllExport, which allows the removal of some custom semantic handling. The same cannot be done for DllImport, and so comments were left explaining why.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196429 91177308-0d34-0410-b5e6-96231b3b80d8