[ClangImporter] Check for API-note-provided attributes by source loc

...rather than by whether they're implicit. We're planning to make
them explicit (prompted by a change in upstream Clang that no longer
prints implicit attributes).

rdar://problem/40346997 (Swift side)

Author: jrose-apple

lib/ClangImporter/ImportName.cpp

@@ -682,9 +682,9 @@ findSwiftNameAttr(const clang::Decl *decl, ImportNameVersion version) {
   auto attr = decl->getAttr<clang::SwiftNameAttr>();
   if (!attr) return nullptr;
 
-  // API notes produce implicit attributes; ignore them because they weren't
-  // used for naming in Swift 2.
-  if (attr->isImplicit()) return nullptr;
+  // API notes produce attributes with no source location; ignore them because
+  // they weren't used for naming in Swift 2.
+  if (attr->getLocation().isInvalid()) return nullptr;
 
   // Hardcode certain kinds of explicitly-written Swift names that were
   // permitted and used in Swift 2. All others are ignored, so that we are