Allow non-ascii-idents in extern blocks #83942
Description
RFC 2457 disallows non-ascii idents in extern blocks and no_mangle.
We added an error for extern blocks in #83923 (no_mangle already errors).
We should figure out if we can allow these -- we'd need to make sure the name mangling schemes consistently handle non-ASCII. I believe at the moment MSVC gets grumpy about this.