std: update std.builtin.Type fields to follow naming conventions

The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.

This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.

test/behavior/usingnamespace.zig

@@ -109,7 +109,7 @@ test "container member access usingnamespace decls" {
 
 usingnamespace opaque {};
 
-usingnamespace @Type(.{ .Struct = .{
+usingnamespace @Type(.{ .@"struct" = .{
     .layout = .auto,
     .fields = &.{},
     .decls = &.{},