[lit] Make builtin cat work with stdin

clang/test/Misc/dev-fd-fs.c

@@ -1,6 +1,5 @@
 // Check that we can operate on files from /dev/fd.
 // REQUIRES: dev-fd-fs
-// REQUIRES: shell
 
 // Check reading from named pipes. We cat the input here instead of redirecting
 // it to ensure that /dev/fd/0 is a named pipe, not just a redirected file.

llvm/utils/lit/lit/builtin_commands/cat.py

@@ -49,6 +49,9 @@ def main(argv):
             import os, msvcrt
 
             msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
+    if len(filenames) == 0:
+        sys.stdout.write(sys.stdin.read())
+        sys.exit(0)
     for filename in filenames:
         try:
             contents = None

llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt

@@ -70,3 +70,7 @@
 # NP-CAT-OUTPUT-NEXT:M-HM-IM-JM-KM-LM-MM-NM-OM-PM-QM-RM-SM-TM-UM-VM-WM-XM-YM-ZM-[
 # NP-CAT-OUTPUT-NEXT:M-\M-]M-^M-_M-`M-aM-bM-cM-dM-eM-fM-gM-hM-iM-jM-kM-lM-mM-nM-o
 # NP-CAT-OUTPUT-NEXT:M-pM-qM-rM-sM-tM-uM-vM-wM-xM-yM-zM-{M-|M-}M-~M-^?
+
+## Test that cat will pipe stdin to stdout if no other files are specified.
+# RUN: echo test | cat | FileCheck --check-prefix=CAT-STDIN %s
+# CAT-STDIN: test