CMake option to allow enabling experimental new pass manager by default

This CMake flag allows setting the default value for the
-f[no]-experimental-new-pass-manager flag.

Differential Revision: https://reviews.llvm.org/D44330

llvm-svn: 329366
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 22263c8..fc7bb78 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -455,7 +455,7 @@
   // Need this flag to turn on new pass manager via Gold plugin.
   if (Args.hasFlag(options::OPT_fexperimental_new_pass_manager,
                    options::OPT_fno_experimental_new_pass_manager,
-                   /* Default */ false)) {
+                   /* Default */ ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER)) {
     CmdArgs.push_back("-plugin-opt=new-pass-manager");
   }