Rename cxx-benchmark-unittests target and convert to LIT.

This patch renames the cxx-benchmark-unittests to check-cxx-benchmarks
and converts the target to use LIT in order to make the tests run faster
and provide better output.

In particular this runs each benchmark in a suite one by one, allowing
more parallelism while ensuring output isn't garbage with multiple threads.

Additionally, it adds the CMake flag '-DLIBCXX_BENCHMARK_TEST_ARGS=<list>'
to specify what options are passed when running the benchmarks.

llvm-svn: 346888
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 336a1a681189bf2b50b7bd0f8229656c58ac1b37
diff --git a/benchmarks/lit.cfg.py b/benchmarks/lit.cfg.py
new file mode 100644
index 0000000..84857d5
--- /dev/null
+++ b/benchmarks/lit.cfg.py
@@ -0,0 +1,23 @@
+# -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
+# Configuration file for the 'lit' test runner.
+import os
+import site
+
+site.addsitedir(os.path.join(os.path.dirname(os.path.dirname(__file__)), 'utils'))
+from libcxx.test.googlebenchmark import GoogleBenchmark
+
+# Tell pylint that we know config and lit_config exist somewhere.
+if 'PYLINT_IMPORT' in os.environ:
+    config = object()
+    lit_config = object()
+
+# name: The name of this test suite.
+config.name = 'libc++ benchmarks'
+config.suffixes = []
+
+config.test_exec_root = os.path.join(config.libcxx_obj_root, 'benchmarks')
+config.test_source_root = config.test_exec_root
+
+config.test_format = GoogleBenchmark(test_sub_dirs='.',
+                                     test_suffix='.libcxx.out',
+                                     benchmark_args=config.benchmark_args)
\ No newline at end of file