Value: add intern and unintern to facilitate code conversion

This allows some code (like struct initializers) to use interned types
while other code (such as comptime mutation) continues to use legacy
types.

With these changes, an `zig build-obj empty.zig` gets to a crash on
missing interned error union types.

src/codegen/c.zig

@@ -566,7 +566,7 @@ pub const DeclGen = struct {
                 try writer.writeAll("){ .ptr = ");
             }
 
-            try dg.renderValue(writer, ty.slicePtrFieldType(mod), val.slicePtr(), .Initializer);
+            try dg.renderValue(writer, ty.slicePtrFieldType(mod), val.slicePtr(mod), .Initializer);
 
             const len_val = try mod.intValue(Type.usize, val.sliceLen(mod));