Require names where we expect binding patterns (#2782)

This also clusters the various `var` tests. This tests similar patterns for a few syntaxes, but `fn f(x: i32, i32) {}` and local variables `var (x: i32, i32);` and `var i32;` were the crashers I found.

In `fn`, the `x: i32,` is helpful to convince the parser that this is valid syntax.

Fixes #2778

explorer/testdata/generic_function/fail_type_only.carbon

@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+class C {}
+
+// CHECK:STDERR: SYNTAX ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_only.carbon:[[@LINE+1]]: syntax error, unexpected RIGHT_SQUARE_BRACKET, expecting COLON
+fn f[x:! i32, addr C]() {}
+
+fn Main() -> i32 {
+  return 0;
+}