Use span_bug for unexpected field projection type
Improves the compiler error backtrace information, as shown in #99363,
by using `span_bug` instead of `bug`.
New output:
```
build/aarch64-apple-darwin/stage1/bin/rustc /tmp/test.rs --edition=2021
error: internal compiler error: compiler/rustc_middle/src/ty/closure.rs:185:25: Unexpected type Opaque(DefId(0:5 ~ test[db0f]::main::T::{opaque#0}), []) for `Field` projection
--> /tmp/test.rs:11:27
|
11 | let Foo((a, b)) = foo;
| ^^^
thread 'rustc' panicked at 'Box<dyn Any>', /Users/jmq/src/forked/rust/compiler/rustc_errors/src/lib.rs:1331:9
stack backtrace:
```
(Remainder of output truncated.)
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