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.

doc/langref/test_variadic_function.zig

@@ -5,7 +5,7 @@ pub extern "c" fn printf(format: [*:0]const u8, ...) c_int;
 
 test "variadic function" {
     try testing.expect(printf("Hello, world!\n") == 14);
-    try testing.expect(@typeInfo(@TypeOf(printf)).Fn.is_var_args);
+    try testing.expect(@typeInfo(@TypeOf(printf)).@"fn".is_var_args);
 }
 
 // test