ThinLTO: never promote as external weak

This linkage is *not* intended to express that a declaration refers
to a weak symbol, but that the symbol might not be present at link
time. I don't believe it was the intent.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266856
diff --git a/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
index 0783075..c0628b3 100644
--- a/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
+++ b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
@@ -132,7 +132,7 @@
     // linker. The module linking caller needs to enforce this.
     assert(!doImportAsDefinition(SGV));
     // If imported as a declaration, it becomes external_weak.
-    return GlobalValue::ExternalWeakLinkage;
+    return SGV->getLinkage();
 
   case GlobalValue::WeakODRLinkage:
     // For weak_odr linkage, there is a guarantee that all copies will be