Lazily construct panic messages in char_lit().

Lazily construct panic messages in char_lit().

This reduces the time taken to run
`rustc -Zparse-only rustc-benchmarks/issue-32278-big-array-of-strings`
from 0.18s to 0.15s on my machine, and reduces the number of
instructions (as measured by Cachegrind) from 1.34B to 1.01B.

With the change applied, the time to fully compile that benchmark is
1.96s, so this is a 1.5% improvement.