Fix leaky diagnostic ignored pragma

[ZedThree]

Ignoring the -Wconversion diagnostic in make_format_args was leaking out of the header, resulting in that warning being ignored in downstream code that includes fmt/base.h:

fmt/include/fmt/base.h

Lines 2823 to 2828 in 9395ef5

	constexpr FMT_ALWAYS_INLINE auto make_format_args(T&... args)
	-> detail::format_arg_store<Context, NUM_ARGS, NUM_NAMED_ARGS, DESC> {
	// Suppress warnings for pathological types convertible to detail::value.
	FMT_PRAGMA_GCC(diagnostic ignored "-Wconversion")
	return {{args...}};
	}

We should push/pop the diagnostics to ensure this is cleaned up (as is done for clang and for GCC options).

See this Compiler Explorer reproducer for the problem. You can see that gcc doesn't raise a warning while clang does. Commenting out the #include <fmt/base.h> restores the correct behaviour.

[vitaut]

Thanks for the PR! The lint issue is preexisting and can be ignored - I'll fix it separately.