[libc++] Allow specifying conditional compile flags dependent on basic Lit features

[ldionne]

Summary

This patch adds support for passing basic Lit features to the
ADDITIONAL_COMPILE_FLAGS keyword by enclosing them in parentheses.
This is done to support https://llvm.org/D131836.

In the future, we should instead add proper support for conditional
keywords in Lit, so that we can evaluate arbitrary Lit boolean
expressions such as ADDITIONAL_COMPILE_FLAGS(x && !y): -flag.

Note that I can see this being exceptionally useful when combined
with RUN commands, which would allow using different commands on
different systems. For example:

RUN(!buildhost=windows): something
RUN(buildhost=windows): something-else

[Mordante]

Thanks for the quick patch! LGTM!

[ldionne]

Try to fix CI on AIX.

[var-const]

Very cool!

libcxx/test/libcxx/selftest/additional_compile_flags/conditional-compile-flags.sh.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test ensures that we can add compile flags that are conditional on Lit features.
+
+// ADDITIONAL_COMPILE_FLAGS(some-defined-feature): -this-flag-should-be-added
+// ADDITIONAL_COMPILE_FLAGS(some-undefined-feature): -this-flag-should-not-be-added
+// RUN: echo "%{compile_flags}" | grep -e '-this-flag-should-be-added'
+// RUN: echo "%{compile_flags}" | grep --invert-match -e '-this-flag-should-not-be-added'

libcxx/test/libcxx/selftest/additional_compile_flags/lit.local.cfg

+# Add a Lit feature so we can test conditional addition of compile flags.
+config.available_features.add('some-defined-feature')

libcxx/utils/libcxx/test/format.py

parsers = [
         lit.TestRunner.IntegratedTestKeywordParser('FILE_DEPENDENCIES:',
                                                    lit.TestRunner.ParserKind.LIST,
                                                    initial_value=fileDependencies),
         lit.TestRunner.IntegratedTestKeywordParser('ADDITIONAL_COMPILE_FLAGS:',
                                                    lit.TestRunner.ParserKind.LIST,
                                                    initial_value=additionalCompileFlags)
     ]
 
+    # Add conditional parsers for ADDITIONAL_COMPILE_FLAGS. This should be replaced by first
+    # class support for conditional keywords in Lit, which would allow evaluating arbitrary
+    # Lit boolean expressions instead.
+    for feature in test.config.available_features:
+        parser = lit.TestRunner.IntegratedTestKeywordParser('ADDITIONAL_COMPILE_FLAGS({}):'.format(feature),
+                                                            lit.TestRunner.ParserKind.LIST,
+                                                            initial_value=additionalCompileFlags)
+        parsers.append(parser)
+
     scriptInTest = lit.TestRunner.parseIntegratedTestScript(test, additional_parsers=parsers,
                                                             require_script=not preamble)
     if isinstance(scriptInTest, lit.Test.Result):
         return scriptInTest
 
     script = []
 
     # For each file dependency in FILE_DEPENDENCIES, inject a command to copy