[libcxx] [test] Fix the _supportsVerify check on Windows by using hasCompileFlag

[mstorsjo]

Summary

The %{cxx} substitution can be quoted with single quotes, which is
fine with unix shells and when executing commands with the lit
internal shell, but breaks when executed directly with subprocess.call()
on Windows.

(The alternative would be to fix the quoting of the subsitution, but
that turns out to be a deeper rabbit hole than expected.)

This also has the side effect of reducing the number of calls to
the compiler, as the hasCompileFlag method is cached.

This unlocks 114 tests that previously were skipped on Windows.

This is a much more straightforward alternative to D103310; it doesn't
address the issue of the way those strings are quoted though, but
avoids that being an issue.

[ldionne]

libcxx/utils/libcxx/test/format.py

18

So this is a little bit tricky. The thing is that the test format is used inside the DSL, and I'd like to avoid a chicken-and-egg problem. IOW, I see this as a layering violation - the testing format is intended to be the base most thing that doesn't have any other dependencies except Lit proper.

[mstorsjo]

libcxx/utils/libcxx/test/format.py

18

Hmm, ok, I see...

FWIW, would it make sense to memoize this function to get it cached? When I had a printout here, I saw quite a number of calls during a regular test run.

[mstorsjo]

Went with D103310 after all, thus this one isn't needed.

libcxx/utils/libcxx/test/format.py

 #===----------------------------------------------------------------------===##
 #
 # 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
 #
 #===----------------------------------------------------------------------===##
 
+from libcxx.test.dsl import *
 import lit
 import lit.formats
 import os
 import pipes
 import re
 import shutil
 import subprocess
 
 def _supportsVerify(config):
     """
     Determine whether clang-verify is supported by the given configuration.
 
     This is done by checking whether the %{cxx} substitution in that
     configuration supports certain compiler flags.
     """
-    command = "%{{cxx}} -xc++ {} -Werror -fsyntax-only -Xclang -verify-ignore-unexpected".format(os.devnull)
-    command = lit.TestRunner.applySubstitutions([command], config.substitutions,
-                                                recursion_limit=config.recursiveExpansionLimit)[0]
-    devNull = open(os.devnull, 'w')
-    result = subprocess.call(command, shell=True, stdout=devNull, stderr=devNull)
-    return result == 0
+    return hasCompileFlag(config, '-Xclang -verify-ignore-unexpected')
 
 def _getTempPaths(test):
     """
     Return the values to use for the %T and %t substitutions, respectively.
 
     The difference between this and Lit's default behavior is that we guarantee
     that %T is a path unique to the test being run.
     """