[MC] Rewrite stdin.s to use python

This test needs to change the file descriptor offset for the llvm-mc
output file to test that we do not get an assertion in that situation.
This doesn't seem easy to do without bash subshells. Rewrite these test
in Python so we can remove the shell requirement from this test and
enable using it with lit's internal shell. This also has the bonus of
making the behavior that we are trying to create for the test much more
explicit (a .seek call on the FD).

Pull Request: llvm#157232

Author: boomanaiden154

llvm/test/MC/COFF/stdin.py

@@ -0,0 +1,30 @@
+# RUN: echo "// comment" > %t.input
+# RUN: which 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,
+    )