Revert of r223763, in spirit.

r223763 was made to work around a temporary issue where a user of the
JIT was passing down a declaration (incorrectly). This shouldn't
occur, so assert rather than silently continue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224277 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Linker/LinkModules.cpp

@@ -1533,8 +1533,7 @@ bool ModuleLinker::run() {
     LazilyLinkGlobalValues.pop_back();
 
     if (auto F = dyn_cast<Function>(SGV))
-      if (F->isDeclaration())
-        continue;
+      assert(!F->isDeclaration() && "users should not pass down decls");
     if (linkGlobalValueBody(*SGV))
       return true;
   }