[PR32667] -Wdocumentation should allow @param/@returns for fields/variables

that have a function/block pointer type

This commit improves the -Wdocumentation warning by making sure that @param and
@returns commands won't trigger warnings when used for fields, variables,
or properties whose type is a function/block pointer type. The
function/block pointer type must be specified directly with the declaration,
and when a typedef is used the warning is still emitted.

In the future we might also want to handle the std::function type as well.

rdar://24978538

llvm-svn: 300981