FunctionImport: Use IRMover directly.

The importer was previously using ModuleLinker in a sort of "IRMover mode". Use
IRMover directly instead in order to remove a level of indirection.

I will remove all importing support from ModuleLinker in a separate
change.

Differential Revision: https://reviews.llvm.org/D29468

llvm-svn: 294014
diff --git a/llvm/lib/Linker/LinkModules.cpp b/llvm/lib/Linker/LinkModules.cpp
index 7ff84db..eb9fdc1 100644
--- a/llvm/lib/Linker/LinkModules.cpp
+++ b/llvm/lib/Linker/LinkModules.cpp
@@ -129,8 +129,7 @@
 bool ModuleLinker::doImportAsDefinition(const GlobalValue *SGV) {
   if (!isPerformingImport())
     return false;
-  return FunctionImportGlobalProcessing::doImportAsDefinition(SGV,
-                                                              GlobalsToImport);
+  report_fatal_error("ModuleLinker does not support importing");
 }
 
 static GlobalValue::VisibilityTypes