Raw ident suggestion should only trigger if the code is otherwise well-formed

[estebank]

Right now the raw ident suggestion when using a reserved keyword somewhere where it is not allowed is context free, leading to misleading output:

https://twitter.com/isislovecruft/status/1191768296590315520

The parser should perform some lookahead to see if it could actually continue parsing if a raw identifier had been supplied in its place and if so, only then suggest raw idents.

[petrochenkov]

Or we shouldn't suggest using raw identifiers if we cannot do that easily and reliably.