[benchmark] Fix buildbots failing to identify regex support

This is cleanup after newly introduced google/benchmark library
(rL340809). Many buildbots fail to identify regex engine support, so
this should presumably fix the issue.

llvm-svn: 340827

llvm/CMakeLists.txt

@@ -1026,6 +1026,8 @@ if (LLVM_INCLUDE_BENCHMARKS)
   set(BENCHMARK_ENABLE_INSTALL OFF CACHE BOOL "Don't install benchmark" FORCE)
   set(BENCHMARK_DOWNLOAD_DEPENDENCIES OFF CACHE BOOL "Don't download dependencies" FORCE)
   set(BENCHMARK_ENABLE_GTEST_TESTS OFF CACHE BOOL "Disable Google Test in benchmark" FORCE)
+  # Since LLVM requires C++11 it is safe to assume that std::regex is available.
+  set(HAVE_STD_REGEX ON CACHE BOOL "OK" FORCE)
 
   add_subdirectory(utils/benchmark)
   add_subdirectory(benchmarks)