[libclang] Don't crash when saving a PCH from a prefix header
that does not exist.

rdar://11607033

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158193 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/FrontendAction.cpp b/lib/Frontend/FrontendAction.cpp
index 1ff3292..fb53c71 100644
--- a/lib/Frontend/FrontendAction.cpp
+++ b/lib/Frontend/FrontendAction.cpp
@@ -315,7 +315,7 @@
   return false;
 }
 
-void FrontendAction::Execute() {
+bool FrontendAction::Execute() {
   CompilerInstance &CI = getCompilerInstance();
 
   // Initialize the main file entry. This needs to be delayed until after PCH
@@ -325,7 +325,7 @@
                                     getCurrentInput().IsSystem
                                       ? SrcMgr::C_System
                                       : SrcMgr::C_User))
-      return;
+      return false;
   }
 
   if (CI.hasFrontendTimer()) {
@@ -333,6 +333,8 @@
     ExecuteAction();
   }
   else ExecuteAction();
+
+  return true;
 }
 
 void FrontendAction::EndSourceFile() {