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/Target/XCore/XCoreAsmPrinter.cpp

@@ -135,10 +135,6 @@ void XCoreAsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
   case GlobalValue::InternalLinkage:
   case GlobalValue::PrivateLinkage:
     break;
-  case GlobalValue::DLLImportLinkage:
-    llvm_unreachable("DLLImport linkage is not supported by this target!");
-  case GlobalValue::DLLExportLinkage:
-    llvm_unreachable("DLLExport linkage is not supported by this target!");
   default:
     llvm_unreachable("Unknown linkage type!");
   }