libstdc++: Add pretty printer for std::initializer_list

libstdc++: Add pretty printer for std::initializer_list

Re-using the std::span printer, this now shows the contents of the
initializer list instead of the pointer and length members.

Signed-off-by: Philipp Fent <fent@in.tum.de>
libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdSpanPrinter._iterator):
	Rename as iterator.
	(StdInitializerListPrinter): Define new printer.
	(build_libstdcxx_dictionary): Register new printer.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Check printer for
	initializer_list.