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/reflection.zig

@@ -6,7 +6,7 @@ const reflection = @This();
 
 test "reflection: function return type, var args, and param types" {
     comptime {
-        const info = @typeInfo(@TypeOf(dummy)).Fn;
+        const info = @typeInfo(@TypeOf(dummy)).@"fn";
         try expect(info.return_type.? == i32);
         try expect(!info.is_var_args);
         try expect(info.params.len == 3);