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().

libstdc++-v3/include/std/source_location

@@ -43,12 +43,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
   private:
     using uint_least32_t = __UINT_LEAST32_TYPE__;
+    using __builtin_ret_type = decltype(__builtin_source_location());
 
   public:
 
     // [support.srcloc.cons], creation
     static consteval source_location
-    current(const void* __p = __builtin_source_location()) noexcept
+    current(__builtin_ret_type __p = __builtin_source_location()) noexcept
     {
       source_location __ret;
       __ret._M_impl = static_cast <const __impl*>(__p);