Improve recovery when users write where:

[compiler-errors]

Improve recovery of where:.

Fixes #143023

The erroneous suggestion was because we were seeing : then a type, which the original impl thought must be a struct field. Make this a bit more accurate by checking for a non-reserved ident (which should be a field name).

Also, make a custom parser error for where: so we can continue parsing after the colon.

[rustbot]

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