[libcxx] [test] Don't rerun supportsVerify for each individual test

We can't just memoize _supportsVerify in place in format.py, as it
previously was executed in each of the individual processes.

Instead use hasCompileFlag() and add a feature flag for it instead,
which can be used both by tests (that already have such a flag,
locally for one set of tests) and for the testing framework itself.

By using hasCompileFlag(), this also implicitly fixes two other issues:
Previously, _supportsVerify called subprocess.call() directly, which can
interpret command line quoting differently than lit.TestRunner.

(In particular, TestRunner handles arguments quoted by a single quote,
while launching Windows processes with subprocess.call() only supports
double quotes. This allows using shlex.quote(), which uses single quotes,
everywhere - as all commands now go through TestRunner. This should make
41d7909368 redundant.)

Secondly, the old _supportsVerify method didn't include %{flags) or
%{compile_flags}.

Differential Revision: https://reviews.llvm.org/D116010

libcxx/test/libcxx/selftest/fail.cpp/lit.local.cfg

@@ -1,6 +0,0 @@
-import libcxx.test.format
-
-# The tests in this directory need to know whether Clang-verify is supported
-# to work properly.
-if libcxx.test.format._supportsVerify(config):
-    config.available_features.add('verify-support')

libcxx/utils/libcxx/test/features.py

@@ -41,6 +41,7 @@ DEFAULT_FEATURES = [
   Feature(name='has-fobjc-arc',                 when=lambda cfg: hasCompileFlag(cfg, '-xobjective-c++ -fobjc-arc') and
                                                                  sys.platform.lower().strip() == 'darwin'), # TODO: this doesn't handle cross-compiling to Apple platforms.
   Feature(name='objective-c++',                 when=lambda cfg: hasCompileFlag(cfg, '-xobjective-c++ -fobjc-arc')),
+  Feature(name='verify-support',                when=lambda cfg: hasCompileFlag(cfg, '-Xclang -verify-ignore-unexpected')),
 
   Feature(name='non-lockfree-atomics',
           when=lambda cfg: sourceBuilds(cfg, """

libcxx/utils/libcxx/test/format.py

@@ -12,21 +12,6 @@ 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
 
 def _getTempPaths(test):
     """
@@ -216,7 +201,7 @@ class CxxStandardLibraryTest(lit.formats.TestFormat):
 
     def execute(self, test, litConfig):
         VERIFY_FLAGS = '-Xclang -verify -Xclang -verify-ignore-unexpected=note -ferror-limit=0'
-        supportsVerify = _supportsVerify(test.config)
+        supportsVerify = 'verify-support' in test.config.available_features
         filename = test.path_in_suite[-1]
 
         # TODO(ldionne): We currently disable tests that re-define _LIBCPP_ASSERT