Adding dllimport, dllexport and external weak linkage types.
DLL* linkages got full (I hope) codegeneration support in C & both x86
assembler backends.
External weak linkage added for future use, we don't provide any
codegeneration, etc. support for it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30374 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Linker/LinkArchives.cpp b/lib/Linker/LinkArchives.cpp
index e115b93..15d261e 100644
--- a/lib/Linker/LinkArchives.cpp
+++ b/lib/Linker/LinkArchives.cpp
@@ -65,16 +65,22 @@
     if (I->hasName()) {
       if (I->isExternal())
         UndefinedSymbols.insert(I->getName());
-      else if (!I->hasInternalLinkage())
+      else if (!I->hasInternalLinkage()) {
+        assert(!I->hasDLLImportLinkage()
+               && "Found dllimported non-external symbol!");
         DefinedSymbols.insert(I->getName());
+      }      
     }
   for (Module::global_iterator I = M->global_begin(), E = M->global_end();
        I != E; ++I)
     if (I->hasName()) {
       if (I->isExternal())
         UndefinedSymbols.insert(I->getName());
-      else if (!I->hasInternalLinkage())
+      else if (!I->hasInternalLinkage()) {
+        assert(!I->hasDLLImportLinkage()
+               && "Found dllimported non-external symbol!");
         DefinedSymbols.insert(I->getName());
+      }      
     }
 
   // Prune out any defined symbols from the undefined symbols set...