[MC] Rewrite stdin.s to use python

llvm/test/MC/COFF/stdin.py

@@ -0,0 +1,32 @@
+# RUN: echo "// comment" > %t.input
+# We use STDIN to for the binary name as lit will substitute in the full path
+# of the binary before executing, ensuring we pick up the correct llvm-mc.
+# RUN: echo llvm-mc | %python %s %t.input %t
+
+import argparse
+import subprocess
+import sys
+
+parser = argparse.ArgumentParser()
+parser.add_argument("input_file")
+parser.add_argument("temp_file")
+arguments = parser.parse_args()
+
+llvm_mc_binary = sys.stdin.readlines()[0].strip()
+
+with open(arguments.temp_file, "w") as mc_stdout:
+    ## We need to test that starting on an input stream with a non-zero offset
+    ## does not trigger an assertion in WinCOFFObjectWriter.cpp, so we seek
+    ## past zero for STDOUT.
+    mc_stdout.seek(4)
+    subprocess.run(
+        [
+            llvm_mc_binary,
+            "-filetype=obj",
+            "-triple",
+            "i686-pc-win32",
+            arguments.input_file,
+        ],
+        stdout=mc_stdout,
+        check=True,
+    )