internal/lsp/source: fix an infinite loop in Deref function

Return a pointer type if the type refers to itself (for example, type a *a).

Fixes golang/go#45510

Author: ShoshinNikita

internal/lsp/source/util.go

@@ -221,12 +221,17 @@ func FormatNode(fset *token.FileSet, n ast.Node) string {
 
 // Deref returns a pointer's element type, traversing as many levels as needed.
 // Otherwise it returns typ.
+//
+// It can return a pointer type if the type refers to itself (see golang/go#45510).
 func Deref(typ types.Type) types.Type {
 	for {
 		p, ok := typ.Underlying().(*types.Pointer)
 		if !ok {
 			return typ
 		}
+		if typ == p.Elem() {
+			return typ
+		}
 		typ = p.Elem()
 	}
 }