AST: Accept @_weakLinked on import decls to force weak linkage of symbols from a module.

The effect of declaring an import `@_weakLinked` is to treat every declaration from the module as if it were declared with `@_weakLinked`. This is useful in environments where entire modules may not be present at runtime. Although it is already possible to instruct the linker to weakly link an entire dylib, a Swift attribute provides a way to declare intent in source code and also opens the door to diagnostics and other compiler behaviors that depend on knowing that all the module's symbols will be weakly linked.

rdar://96098097

lib/Sema/ImportResolution.cpp

@@ -554,6 +554,9 @@ UnboundImport::UnboundImport(ImportDecl *ID)
     import.options |= ImportFlags::Preconcurrency;
     import.preconcurrencyRange = attr->getRangeWithAt();
   }
+
+  if (auto attr = ID->getAttrs().getAttribute<WeakLinkedAttr>())
+    import.options |= ImportFlags::WeakLinked;
 }
 
 bool UnboundImport::checkNotTautological(const SourceFile &SF) {