The [cmath.syn] p1 wording about additional overloads sufficient to
handle any arithmetic types also applies to std::lerp. This adds a new
overload of std::lerp that does the required promotions to support
arguments of arbitrary arithmetic types.

A new __promoted_t alias template is added, which the C++17 function
templates std::hypot and std::lerp can use to avoid instantiating the
__promote_3 class template.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/101870
	* include/c_global/cmath (hypot): Use __promoted_t.
	(lerp): Add new overload accepting any arithmetic types.
	* include/ext/type_traits.h (__promoted_t): New alias template.
	* testsuite/26_numerics/lerp.cc: Moved to...
	* testsuite/26_numerics/lerp/1.cc: ...here.
	* testsuite/26_numerics/lerp/constexpr.cc: New test.
	* testsuite/26_numerics/lerp/version.cc: New test.

Tested powerpc64le-linux. Committed to trunk.