Add a FunctionImporter helper to perform summary-based cross-module function importing

Summary:
This is a helper to perform cross-module import for ThinLTO. Right now
it is importing naively every possible called functions.

Reviewers: tejohnson

Subscribers: dexonsmith, llvm-commits

Differential Revision: http://reviews.llvm.org/D14914

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 253954
diff --git a/llvm/lib/Transforms/IPO/IPO.cpp b/llvm/lib/Transforms/IPO/IPO.cpp
index 97e4584..379af30 100644
--- a/llvm/lib/Transforms/IPO/IPO.cpp
+++ b/llvm/lib/Transforms/IPO/IPO.cpp
@@ -48,6 +48,7 @@
   initializeBarrierNoopPass(Registry);
   initializeEliminateAvailableExternallyPass(Registry);
   initializeSampleProfileLoaderPass(Registry);
+  initializeFunctionImportPassPass(Registry);
 }
 
 void LLVMInitializeIPO(LLVMPassRegistryRef R) {