Decouple dllexport/dllimport from linkage

Representing dllexport/dllimport as distinct linkage types prevents using
these attributes on templates and inline functions.

Instead of introducing further mixed linkage types to include linkonce and
weak ODR, the old import/export linkage types are replaced with a new
separate visibility-like specifier:

  define available_externally dllimport void @f() {}
  @Var = dllexport global i32 1, align 4

Linkage for dllexported globals and functions is now equal to their linkage
without dllexport. Imported globals and functions must be either
declarations with external linkage, or definitions with
AvailableExternallyLinkage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199218 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -1211,9 +1211,7 @@ void ExecutionEngine::emitGlobals() {
         }
 
         // If the existing global is strong, never replace it.
-        if (GVEntry->hasExternalLinkage() ||
-            GVEntry->hasDLLImportLinkage() ||
-            GVEntry->hasDLLExportLinkage())
+        if (GVEntry->hasExternalLinkage())
           continue;
 
         // Otherwise, we know it's linkonce/weak, replace it if this is a strong