[llvm-branch-commits] [Clang] Make rewrite-includes-bom.c work with internal shell (PR #158463)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/158463

This test was using $'<content with escapes' to have bash interpret
shell escapes and then passing them to grep. The lexer/parser for lit's
internal shell does not support such sequences. Given this is the only
test in-tree that uses this functionality, it makes much more sense to
use a POSIX compliant utility like od to transform the file into hex and
then grep over that rather than modify the internal shell. This test
should remain portable as od should be widely available and we are only
using options specified in POSIX.