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/Pdb.zig

@@ -495,7 +495,7 @@ const MsfStream = struct {
     blocks: []u32 = undefined,
     block_size: u32 = undefined,
 
-    pub const Error = @typeInfo(@typeInfo(@TypeOf(read)).Fn.return_type.?).ErrorUnion.error_set;
+    pub const Error = @typeInfo(@typeInfo(@TypeOf(read)).@"fn".return_type.?).error_union.error_set;
 
     fn init(block_size: u32, file: File, blocks: []u32) MsfStream {
         const stream = MsfStream{