Enable PCH by default

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70339 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index d6a046e..6f75d3e 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -47,7 +47,7 @@
     Host(0),
     CCCIsCXX(false), CCCEcho(false), CCCPrintBindings(false),
     CCCGenericGCCName("gcc"), CCCUseClang(true), CCCUseClangCXX(false), 
-    CCCUseClangCPP(true), CCCUsePCH(false),
+    CCCUseClangCPP(true), CCCUsePCH(true),
     SuppressMissingInputWarning(false)
 {
   // Only use clang on i386 and x86_64 by default.