[AST] Handle missing @ location in AtTypeAttrBase

Attributes like `nonisolated` aren't spelled with an `@`. I don't
have a test case, but this would assert if the source range were
queried.

include/swift/AST/Attr.h

@@ -3841,7 +3841,9 @@ public:
 
   SourceLoc getAtLoc() const { return AtLoc; }
 
-  SourceLoc getStartLocImpl() const { return AtLoc; }
+  SourceLoc getStartLocImpl() const {
+    return AtLoc.isValid() ? AtLoc : getAttrLoc();
+  }
   SourceLoc getEndLocImpl() const { return getAttrLoc(); }
 };