Add a FIXME comment; NFC

The constant expression engines use a list of note diagnostics to
report back whether an evaluation is a valid constant expression or
not. This requires the engines to generate those note diagnostics, and
that can be expensive in cases where we're converting an APValue to a
string as a streamed argument to the note. If we're going to display
the note, then that expense is warranted. However, there are operations
looking for a yes/no answer to whether something is a constant
expression and won't display the diagnostics at all. Those uses are
paying an expense they shouldn't have to.