libstdc++: Work around modules ICE in <charconv> [PR105297]

This makes the initializer for __table in __from_chars_alnum_to_val
dependent in an artificial way, which works around the reported modules
testsuite ICE by preventing the compiler from evaluating the initializer
parse time.

Compared to the alternative workaround of using a non-local class type
for __table, this workaround has the advantage of slightly speeding up
compilation of <charconv>, since now the table won't get built (via
constexpr evaluation) until the integer std::from_chars overload is
instantiated.

	PR c++/105297
	PR c++/105322

libstdc++-v3/ChangeLog:

	* include/std/charconv (__from_chars_alnum_to_val): Make
	initializer for __table dependent in an artificial way.