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/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 45546ac..1a3b67f 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -526,7 +526,7 @@
 
   Opts.ExperimentalNewPassManager = Args.hasFlag(
       OPT_fexperimental_new_pass_manager, OPT_fno_experimental_new_pass_manager,
-      /* Default */ false);
+      /* Default */ ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER);
 
   Opts.DebugPassManager =
       Args.hasFlag(OPT_fdebug_pass_manager, OPT_fno_debug_pass_manager,