Reapply "[Clang] Make rewrite-includes-bom.c work with internal shell"

This reverts commit 354ad55.

This was reverted previously for causing test failures on MacOS. It turns out
that od -t x1 prints two spaces between hexadecimal representations of bytes
rather than one like on other *nix systems. This reland updates the original
patch to match any number of whitespace characters in between the bytes rather
than just a single space.

Author: boomanaiden154

clang/test/Frontend/rewrite-includes-bom.c

@@ -1,8 +1,7 @@
-// RUN: grep -q $'^\xEF\xBB\xBF' %S/Inputs/rewrite-includes-bom.h
+// RUN: cat %S/Inputs/rewrite-includes-bom.h | od -t x1 | grep -q 'ef\s*bb\s*bf'
 // RUN: %clang_cc1 -E -frewrite-includes -I %S/Inputs %s -o %t.c
-// RUN: ! grep -q $'\xEF\xBB\xBF' %t.c
+// RUN: cat %t.c | od -t x1 | not grep -q 'ef\s*bb\s*bf'
 // RUN: %clang_cc1 -fsyntax-only -verify %t.c
 // expected-no-diagnostics
-// REQUIRES: shell
 
 #include "rewrite-includes-bom.h"