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/Transforms/IPO/Internalize.cpp

@@ -116,7 +116,7 @@ static bool shouldInternalize(const GlobalValue &GV,
     return false;
 
   // Assume that dllexported symbols are referenced elsewhere
-  if (GV.hasDLLExportLinkage())
+  if (GV.hasDLLExportStorageClass())
     return false;
 
   // Already has internal linkage