[ThinLTO] Recommit of import global variables

This wasreverted in r326638 due to link problems and fixed
afterwards

llvm-svn: 327254
diff --git a/llvm/lib/Linker/IRMover.cpp b/llvm/lib/Linker/IRMover.cpp
index ec15bbf..42081442 100644
--- a/llvm/lib/Linker/IRMover.cpp
+++ b/llvm/lib/Linker/IRMover.cpp
@@ -1051,14 +1051,10 @@
     ValueMap.MD()[CU->getRawEnumTypes()].reset(nullptr);
     ValueMap.MD()[CU->getRawMacros()].reset(nullptr);
     ValueMap.MD()[CU->getRawRetainedTypes()].reset(nullptr);
-    // If we ever start importing global variable defs, we'll need to
-    // add their DIGlobalVariable to the globals list on the imported
-    // DICompileUnit. Confirm none are imported, and then we can
-    // map the list of global variables to nullptr.
-    assert(none_of(
-               ValuesToLink,
-               [](const GlobalValue *GV) { return isa<GlobalVariable>(GV); }) &&
-           "Unexpected importing of a GlobalVariable definition");
+    // We import global variables only temporarily in order for instcombine
+    // and globalopt to perform constant folding and static constructor
+    // evaluation. After that elim-avail-extern will covert imported globals
+    // back to declarations, so we don't need debug info for them.
     ValueMap.MD()[CU->getRawGlobalVariables()].reset(nullptr);
 
     // Imported entities only need to be mapped in if they have local