…ng tag helpers (dotnet#10331)

Because of how we were rewriting tag helper attribute values, we didn't output a single, unified token when moving an implicit C# transition back to standard C#. This meant that, when we emit `#line` directives, we considered the `@` and the identifier that followed it to be separate tokens; when we added more precise info for FUSE, this then meant that they were emitted on separate lines during runtime codegen and everything fell apart. To fix this, we now unify those implicit transition tokens, rather than trying to keep them as separate tokens. Fixes dotnet#10186, for real this time.

(cherry picked from commit 26ce354)