Isolate zero_whitespace parser test into its own file.

[silvas]

Summary

This test is in a different file because it contains a literal NUL
character, which causes various tools to treat it as a binary file.
Hence it is useful to have this test kept in a separate, rarely-changing
file.

[silvas]

zero_whitespace.mlir looks like this:

// RUN: mlir-opt %s | FileCheck %s

// This test is in a different file because it contains a literal NUL
// character, which causes various tools to treat it as a binary file.
// Hence it is useful to have this test kept in a separate, rarely-changing
// file.

// CHECK-LABEL: func @zero_whitespace() {
// CHECK-NEXT: return
func @zero_whitespace() {
  ^@  // This is a \0 byte.
  return
}

mlir/test/IR/parser.mlir

func @pretty_names() {
   // CHECK-NOT: attributes
 
   %q:3, %r = test.string_attr_pretty_name
   // CHECK: %q, %q_1, %q_2, %r = test.string_attr_pretty_name attributes {names = ["q", "q", "q", "r"]}
 
   // CHECK: return
   return
 }
-
-// CHECK-LABEL: func @zero_whitespace() {
-// CHECK-NEXT: return
-func @zero_whitespace() {
-  ␀  // This is a \0 byte.
-  return
-}