Auto merge of #41990 - qnighy:disallow-underscore-suffix-for-string-like-literals...

Auto merge of #41990 - qnighy:disallow-underscore-suffix-for-string-like-literals, r=nikomatsakis

Disallow underscore suffix for string-like literals.

This patch turns string/bytestring/char/byte literals followed by an underscore, like `"Foo"_`, to an error.

`scan_optional_raw_name` will parse `_` as a valid raw name, but it will be rejected by the parser. I also considered just stopping parsing when the suffix is `_`, but in that case `"Foo"_` will be lexed as two valid tokens.

Fixes the latter half of #41723.