libstdc++: Redefine __from_chars_alnum_to_val's table

After the C++23 constexpr <charconv> patch r13-3313-g378a0f1840e694 we
have some modules testsuite regressions:

  FAIL: g++.dg/modules/xtreme-header-4_b.C -std=c++2b (test for excess errors)
  FAIL: g++.dg/modules/xtreme-header_b.C -std=c++2b (test for excess errors)

Like with PR105297, the cause seems to be the deduced type of __table
resolving ahead of time to a local class type, which trips up modules.
And unfortunately that PR's minimal workaround of making __tables's
initializer value dependent doesn't help in this case.

So this patch works around this by avoiding using a local class for the
table type.  And I suppose we should use a static data member to define
the table once for all dialects (including C++14) instead of having to
define it twice in C++23 mode (once as a static local variable and again
as a variable template for sake of constexpr evaluation).

libstdc++-v3/ChangeLog:

	* include/std/charconv (__detail::__from_chars_alnum_to_val_table):
	Redefine as a class template containing the members type, value and
	_S_make_table.  Don't use a local class as the table type.
	(__detail::__table): Remove.
	(__detail::__from_chars_alnum_to_val): Adjust after the above.

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