Revert tearing changes to dbg!

[cuviper]

Since the primary change to dbg! in #149869, we've been chasing a few regressions:

• 1.95 beta regression: "temporary value dropped while borrowed" #153850, fixed by don't drop arguments' temporaries in dbg! #154074
• 1.96 beta regression: "lua does not live long enough" #154988, fixed by don't leak internal temporaries from dbg! #154994
• 1.96 beta regression: "lua does not live long enough", take 2 #155902, proposed fix in don't leak internal temporaries from dbg!, even on false unwind paths #155915

We already reverted this once, on beta only to prevent these regressions from shipping in 1.95.

In that most recent PR, we decided that it would be better to revert dbg! to its original state everywhere (main and 1.96-beta), and then we can consider it from scratch later. So here I've reverted the change and its fixes, but kept the regression tests, including the pending one.

cc @joboet @dianne @rust-lang/libs
@rustbot label beta-nominated

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[cuviper]

r? libs