Don't emit debug info for value witness tables.

Value witness tables have a well-known symbol name, so there is nothing
that the debug info adds on top of it.

On top of that, the DWARF type we previously were emitting them with was
bogus.

rdar://problem/21777112

test/DebugInfo/enum.swift

@@ -1,6 +1,15 @@
 // RUN: %target-swift-frontend -primary-file %s -emit-ir -g -o - | FileCheck %s
 
 // CHECK: ![[EMPTY:.*]] = !{}
+
+enum Either {
+  case First(Int64), Second(String), Neither
+// CHECK: !DICompositeType(tag: DW_TAG_union_type, name: "Either",
+// CHECK-SAME:             line: [[@LINE-3]],
+// CHECK-SAME:             size: 200,
+}
+let E : Either = .Neither;
+
 // CHECK: !DICompositeType(tag: DW_TAG_union_type, name: "Color",
 // CHECK-SAME:             line: [[@LINE+3]]
 // CHECK-SAME:             size: 8, align: 8,