[clang-tidy] Fix assert in modernize-use-std-format/print (#94104)

Ensure that FormatStringConverter's constructor fails with a sensible
error message rather than asserting if the format string is not a narrow
string literal.

Also, ensure that we don't even get that far in modernize-use-std-print
and modernize-use-std-format by checking that the format string
parameter is a char pointer.

Fixes #92896