libstdc++: Improve diagnostics for invalid std::format calls

libstdc++: Improve diagnostics for invalid std::format calls

Add a static_assert and a comment so that calling std::format for
unformattable argument types will now show:

/home/jwakely/gcc/13/include/c++/13.0.1/format:3563:22: error: static assertion failed: std::formatter must be specialized for each format arg
 3563 |       static_assert((is_default_constructible_v<formatter<_Args, _CharT>> && ...),
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and:

  140 |       formatter() = delete; // No std::formatter specialization for this type.

libstdc++-v3/ChangeLog:

	* include/std/format (formatter): Add comment to deleted default
	constructor of primary template.
	(_Checking_scanner): Add static_assert.