compiler: un-implement ziglang#19634

This commit reverts the handling of partially-undefined values in
bitcasting to transform these bits into an arbitrary numeric value,
like happens on `master` today.

As @andrewrk rightly points out, ziglang#19634 has unfortunate consequences
for the standard library, and likely requires more thought. To avoid
a major breaking change, it has been decided to revert this design
decision for now, and make a more informed decision further down the
line.

Author: mlugg

compile_errors/bitcast_undef.zig

@@ -4,17 +4,9 @@ export fn entry1() void {
     @compileLog(y);
 }
 
-export fn entry2() void {
-    const x: packed struct { x: u16, y: u16 } = .{ .x = 123, .y = undefined };
-    const y: u32 = @bitCast(x);
-    @compileLog(y);
-}
-
 // error
 //
 // :4:5: error: found compile log statement
-// :10:5: note: also here
 //
 // Compile Log Output:
 // @as(u32, undefined)
-// @as(u32, undefined)