Adding manual headings to the new capability attributes' documentation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201942 91177308-0d34-0410-b5e6-96231b3b80d8
Reorganize and improve semantic tests for dllexport/import

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201947 91177308-0d34-0410-b5e6-96231b3b80d8