Remove libdebug and update tests.
libsyntax: Remove all uses of {:?}.
librustc: Remove all uses of {:?}.
libstd: Remove all uses of {:?}.
liballoc: Remove all uses of {:?}.
librustdoc: Remove all uses of {:?}.
libcollections: Remove all uses of {:?}.
libnative: Remove all uses of {:?}.
librustc: Remove visit_tydesc intrinsic.
librustc: Stop generating visit glue and remove from TyDesc.
librustc: Remove trans::reflect and the opaque and ty_visitor lang items.
libsyntax: Don't accept :? as a format specifier.
librustc: Remove special case for old reflection visitor.
auto merge of #18064 : luqmana/rust/remove-reflection, r=nick29581

Out goes reflection! This means your code will break if you used the `:?` format specifier, anything else from libdebug, or the `visit_tydesc` intrinsic directly.

Closes #18046.

[breaking-change]