[llvm] [lit] Run builtin cat / diff in-process instead of spawning (PR #208024)

[Prasoon Kumar via llvm-commits]

================
@@ -206,6 +207,58 @@ def processRedirects(cmd, stdin_source, cmd_shenv, opened_files):
     return std_fds
 
 
+def as_binary_reader(stream):
----------------
prasoon054 wrote:

Yes, will add type hints here.

https://github.com/llvm/llvm-project/pull/208024