[llvm-lit] Lit's built-in cat command not handling carriage returns correctly

[connieyzhu]

When using lit's internal shell to run clang-tools-extra/test/clang-apply-replacements/crlf.cpp, the test fails. Upon further inspection, this is due to lit's built-in cat command not behaving the same as many systems' cat (e.g. Linux). It is not able to identify carriage returns (^M) in files.

This problem was diagnosed by running cat %S/Inputs/crlf/crlf.cpp (in line 2 of the above file) with both the internal shell and with a Linux shell, and redirecting output to two different files. When analyzing the files with cat -e in the Linux shell, the following is displayed:

cat %S/Inputs/crlf/crlf.cpp run with Linux shell (expected output):

^M$
// This file intentionally uses a CRLF newlines!^M$
^M$
void foo() {^M$
  int *x = 0;^M$
}^M$

cat %S/Inputs/crlf/crlf.cpp run with lit internal shell (actual output):

$
// This file intentionally uses a CRLF newlines!$
$
void foo() {$
  int *x = 0;$
}$

This poses an issue, as the tests in this file rely on precise character offset to make text replacements.