libstdc++: Make __from_chars_alnum_to_val conversion explicit

The optimizations from commit r12-8175-ga54137c88061c7 introduced a
clang integer sanitizer error.

Fix this with an explicit static_cast, similar to the fix for PR 96766.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_alnum_to_val): Replace
	implicit conversion from int to unsigned char with explicit
	cast.

libstdc++-v3/include/std/charconv

@@ -436,7 +436,7 @@ namespace __detail
     __from_chars_alnum_to_val(unsigned char __c)
     {
       if _GLIBCXX17_CONSTEXPR (_DecOnly)
-	return __c - '0';
+	return static_cast<unsigned char>(__c - '0');
       else
 	{
 	  // This initializer is deliberately made dependent in order to work