libstdc++: Fix cast in source_location::current() [PR104602]

libstdc++: Fix cast in source_location::current() [PR104602]

This fixes a problem for Clang, which is going to return a non-void
pointer from __builtin_source_location(). The current definition of
std::source_location::current() converts that to void* and then has to
cast it back again in the body (which makes it invalid in a constant
expression). By using the actual type of the returned pointer, we avoid
the problematic cast for Clang.

libstdc++-v3/ChangeLog:

	PR libstdc++/104602
	* include/std/source_location (source_location::current): Use
	deduced type of __builtin_source_location().