Rollup merge of #147413 - karolzwolak:extern-multiple-quotes, r=fmease

don't panic on extern with just multiple quotes in the name

Continues rust-lang/rust#147377.
That PR fixed ICE when the extern name was a single quote `"'"`, but multiple quotes like `"''"` cause the same problem.
I had a random revelation that the trimming can remove more than one quote.
r? ``@nnethercote``

Author: matthiaskrgr