Let inlining report compiletime.error text

[som-snytt]

After typer, add error diagnostics to a special LateMismatchedImplicit, so that inlining can add the text to the "missing arg" message. (It piggy-backs on the otherwise unused "location" string. The existing English grammar works out okay.) This limits the "surface area" of this channel.

The current reference does not suggest that error works only at typer phase:

The error method is used to produce user-defined compile errors during inline expansion.

The reference is liberal about how the user text is represented:

If an inline expansion results in a call error(msgStr) the compiler produces an error message containing the given msgStr.

This commit is more conservative than nicolasstucki's fix for macro errors, since existing clients can ignore the special MismatchedImplicit type, and msg formatting is unchanged.

Fixes #15788