libstdc++: Fix std::format("{:F}", inf) to use uppercase

std::format was treating {:f} and {:F} identically on the basis that for
the fixed 1.234567 format there are no alphabetical characters that need
to be in uppercase. But that's wrong for infinities and NaNs, which
should be formatted as "INF" and "NAN" for {:F}.

libstdc++-v3/ChangeLog:

	* include/std/format (__format::_Pres_type): Add _Pres_F.
	(__formatter_fp::parse): Use _Pres_F for 'F'.
	(__formatter_fp::format): Set __upper for _Pres_F.
	* testsuite/std/format/functions/format.cc: Check formatting of
	infinity and NaN for each presentation type.