This patch fixes a bug that caused the compiler to issue the error "default
iterator must be unique" when one of the alleged "duplicates" is overridden by
another. This can happen in cases involving types derived from types declared
in generic formal packages. The error message (when it is correct) is also
improved to mention where the duplicate declarations occur. No small test is
available.

Tested on x86_64-pc-linux-gnu, committed on trunk

2015-10-26  Bob Duff  <duff@adacore.com>

	* sem_ch13.adb (Check_Iterator_Functions): For a Default_Iterator
	aspect, make sure an implicitly declared interpretation is
	overridden by an explicit one.