c-family: Prevent -Wformat warnings with u8 strings [PR105626]

c-family: Prevent -Wformat warnings with u8 strings [PR105626]

The <https://gcc.gnu.org/pipermail/gcc/2022-May/238679.html> thread
seems to have concluded that -Wformat shouldn't warn about

  printf((const char*) u8"test %d\n", 1);

saying "format string is not an array of type 'char'".  This code
is not an aliasing violation, and there are no I/O functions for u8
strings, so the const char * cast is OK and shouldn't be disregarded.

	PR c++/105626

gcc/c-family/ChangeLog:

	* c-format.cc (check_format_arg): Don't emit -Wformat warnings with
	u8 strings.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wformat-char8_t-1.C: New test.