[NFC] Heavily refactor NameBinding

This commit refactors NameBinding.cpp virtually to the point of a rewrite. The goal is to make the import handling code process a struct containing information extracted from an ImportDecl, rather than the ImportDecl itself, so that a future commit can perform the same processing on imports that are not directly represented by an ImportDecl. The result takes more code, but it disentangles a knot of complicated logic into separate threads.

One semi-functional change is that validation of scoped imports (i.e. checking that the declaration actually exists) is now deferred until all of the file’s imports have been processed. Once cross-imports are supported, this will be necessary because scoped imports can select declarations from cross-import overlays, and the full set of cross-import overlays can’t be known until all imports have been seen.

Some comments refer to things that won’t exist until the next commit, like the visibleModules property. I’ll let you in on a litlte secret: I didn’t really do this all in one go.

Author: beccadax

include/swift/AST/Attr.h

@@ -2103,6 +2103,15 @@ class DeclAttributes {
     return nullptr;
   }
 
+  /// Retrieve the first attribute with the given kind.
+  DeclAttribute *getAttribute(DeclAttrKind DK,
+                              bool AllowInvalid = false) {
+    for (auto Attr : *this)
+      if (Attr->getKind() == DK && (Attr->isValid() || AllowInvalid))
+        return Attr;
+    return nullptr;
+  }
+
 private:
   /// Predicate used to filter MatchingAttributeRange.
   template <typename ATTR, bool AllowInvalid> struct ToAttributeKind {