Make printers use singular noun for a single element

	* python/libstdcxx/v6/printers.py (num_elements): New function.
	(StdMapPrinter.to_string, StdSetPrinter.to_string)
	(StdDequePrinter.to_string, Tr1UnorderedSetPrinter.to_string)
	(Tr1UnorderedMapPrinter.to_string): Use num_elements.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Adjust expected results
	to use singular noun when there is only one element.
	* testsuite/libstdc++-prettyprinters/debug.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/debug_cxx11.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/simple.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/simple11.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/tr1.cc: Likewise.

From-SVN: r243652