[driver] When creating the compiler invocation out of command-line
arguments, force use of clang frontend for the driver.

Fixes rdar://11356765.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157205 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 9340ba1..9e3ba84 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -59,7 +59,7 @@
     CCPrintOptions(false), CCPrintHeaders(false), CCLogDiagnostics(false),
     CCGenDiagnostics(false), CCCGenericGCCName(""), CheckInputsExist(true),
     CCCUseClang(true), CCCUseClangCXX(true), CCCUseClangCPP(true),
-    CCCUsePCH(true), SuppressMissingInputWarning(false) {
+    ForcedClangUse(false), CCCUsePCH(true), SuppressMissingInputWarning(false) {
   if (IsProduction) {
     // In a "production" build, only use clang on architectures we expect to
     // work.