[libc++] money_get::do_get() set failbit and eofbit if iterator begin equals end

Summary:
Currently, if we pass in the same iterator for begin and end,
the long double version of do_get would throw a runtime error.

However, according to standard (https://eel.is/c++draft/locale.money.get#virtuals-1),
we should set the failbit and eofbit when no more characters are available.

Differential Revision: https://reviews.llvm.org/D100510