[libc++] Build Google Benchmark directly from Lit

[ldionne]

This removes a weird interconnection between libc++'s own build and the configuration of the test suite, which should be disjoint. After this patch, Google Benchmark gets built using the same flags used for the rest of the test suite, which provides additional flexibility.

[github-actions]

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darker --check --diff -r origin/main...HEAD libcxx/utils/libcxx/test/config.py libcxx/utils/libcxx/test/format.py

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View the diff from darker here.

--- format.py	2025-09-23 18:52:07.000000 +0000
+++ format.py	2025-09-23 18:55:10.036861 +0000
@@ -346,19 +346,28 @@
                         test.getFullName()
                     ),
                 )
             # TODO: %{libcxx-dir} is not a base substitution
             # TODO: We're building Google Benchmark for every test, which is super wasteful
-            configure = ['cmake', '-S', '%{libcxx-dir}/../third-party/benchmark', '-B', '%T/gbench-build']
-            configure += ['-D', 'CMAKE_INSTALL_PREFIX=%T/gbench']
-            configure += ['-D', 'CMAKE_CXX_COMPILER=%{cxx}']
-            configure += ['-D', 'CMAKE_BUILD_TYPE=Release']
-            configure += ['-D', 'CMAKE_CXX_FLAGS="%{flags} %{compile_flags} %{link_flags} -Wno-error"']
-            configure += ['-D', 'BENCHMARK_ENABLE_TESTING=OFF']
-            configure += ['-D', 'BENCHMARK_ENABLE_WERROR=OFF']
-            configure += ['-D', 'BENCHMARK_INSTALL_DOCS=OFF']
-            build = ['cmake', '--build', '%T/gbench-build', '--target', 'install']
+            configure = [
+                "cmake",
+                "-S",
+                "%{libcxx-dir}/../third-party/benchmark",
+                "-B",
+                "%T/gbench-build",
+            ]
+            configure += ["-D", "CMAKE_INSTALL_PREFIX=%T/gbench"]
+            configure += ["-D", "CMAKE_CXX_COMPILER=%{cxx}"]
+            configure += ["-D", "CMAKE_BUILD_TYPE=Release"]
+            configure += [
+                "-D",
+                'CMAKE_CXX_FLAGS="%{flags} %{compile_flags} %{link_flags} -Wno-error"',
+            ]
+            configure += ["-D", "BENCHMARK_ENABLE_TESTING=OFF"]
+            configure += ["-D", "BENCHMARK_ENABLE_WERROR=OFF"]
+            configure += ["-D", "BENCHMARK_INSTALL_DOCS=OFF"]
+            build = ["cmake", "--build", "%T/gbench-build", "--target", "install"]
 
             steps = [
                 "rm -rf %T/gbench %T/gbench-build",
                 "%dbg(CONFIGURING GBENCH) " + " ".join(configure),
                 "%dbg(BUILDING GBENCH) " + " ".join(build),