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.

lib/std/debug.zig

@@ -547,7 +547,7 @@ pub fn writeStackTrace(
 }
 
 pub const UnwindError = if (have_ucontext)
-    @typeInfo(@typeInfo(@TypeOf(StackIterator.next_unwind)).Fn.return_type.?).ErrorUnion.error_set
+    @typeInfo(@typeInfo(@TypeOf(StackIterator.next_unwind)).@"fn".return_type.?).error_union.error_set
 else
     void;