Revert "Disallow ambiguous attributes on expressions" on nightly
As discussed in [today's t-compiler meeting](https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202024-06-06/near/443079505), this reverts PR #124099 to fix P-critical beta regressions #125199.
r? ``@wesleywiser``
Opening as draft so that ``@wesleywiser`` and ``@apiraino,`` you can tell me whether you wanted:
1. a `beta-accepted` revert of #124099 on nightly (this PR)? That will need to be backported to beta (even though #126093 may be the last of those)
2. a revert of #124099 on beta?
3. all of the above?
I also opened #126102, another draft PR to revert #124099 on beta, should you choose options 2 or 3.
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