[MC] Rewrite stdin.s to use python

llvm/test/MC/COFF/stdin.py

@@ -0,0 +1,25 @@
+# RUN: echo "// comment" > %t.input
+# RUN: which llvm-mc | %python %s %t
+
+import argparse
+import subprocess
+import sys
+
+parser = argparse.ArgumentParser()
+parser.add_argument("temp_file")
+arguments = parser.parse_args()
+
+llvm_mc_binary = sys.stdin.readlines()[0].strip()
+temp_file = arguments.temp_file
+input_file = temp_file + ".input"
+
+with open(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", input_file],
+        stdout=mc_stdout,
+        check=True,
+    )