[Sema] Fix diagnostics for one-byte length modifier

In case a char-literal of type int (C/ObjectiveC) corresponds to a
format specifier with the %hh length modifier, don't treat the literal
as of type char for issuing diagnostics, as otherwise this results in:

printf("%hhd", 'e');
warning: format specifies type 'char' but the argument has type 'char'.

Differential revision: https://reviews.llvm.org/D97951