[libcxx][lit] Add a new executor that only compiles tests

[arichardson]

Summary

This executor is very useful when you are running tests on a slow
(single-threaded) QEMU emulator via ssh (as is the case in our CHERI fork)
since you can't use the -j flag of lit to speed things up.
In the past I have used this as a basic regression test when adding new
compiler features to check that no new LLVM assertions are triggered.

I originally added this executor for the old test format to the CHERI fork
of libc++. This change is a simple port of the executor to the new format.
I have since added wrapper scripts that start multiple QEMU instances and
distribute the testcases across these instances, but this executor is still
useful and in our CI jobs we use this as an first-stage executor for
libunwind/libcxx to avoid the long-running testing via SSH when there are
obvious bugs in the compiler.

[ldionne]

This is an interesting idea. What do you think of https://reviews.llvm.org/D77657?

[arichardson]

In D84052#2159015, @ldionne wrote:

This is an interesting idea. What do you think of https://reviews.llvm.org/D77657?

I think D77657 looks like a very useful idea. It looks to me like it's a better version of my simplistic D84045.

[arichardson]

Not particularly useful.

libcxx/utils/compile_only.py

+#!/usr/bin/env python3
+#===----------------------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+
+"""compile_only.py is a utility for simulating running of a program."""
+
+import argparse
+import subprocess
+import sys
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--execdir', type=str, required=True)
+    parser.add_argument('--codesign_identity', type=str, required=False, default=None)
+    parser.add_argument('--env', type=str, nargs='*', required=False, default=dict())
+    (args, remaining) = parser.parse_known_args(sys.argv[1:])
+
+    if len(remaining) < 2:
+        sys.stderr.write('Missing actual commands to run')
+        exit(1)
+    commandLine = remaining[1:] # Skip the '--'
+
+    # Do any necessary codesigning.
+    if args.codesign_identity:
+        exe = commandLine[0]
+        rc = subprocess.call(['xcrun', 'codesign', '-f', '-s', args.codesign_identity, exe], env={})
+        if rc != 0:
+            sys.stderr.write('Failed to codesign: ' + exe)
+            return rc
+
+    # Extract environment variables into a dictionary
+    env = {k : v  for (k, v) in map(lambda s: s.split('=', 1), args.env)}
+    # Run the command line with the given environment in the execution directory.
+    print("Would have run", subprocess.list2cmdline(commandLine), "in", args.execdir, "with env=", env)
+
+
+if __name__ == '__main__':
+    exit(main())