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

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

To display (o-,i-)stringstreams in the common case, we just print the
underlying stringbuf, without the many ios_base members. In the
unconventional case that the underlying streambuf was redirected, we
report the redirected target.

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

	* python/libstdcxx/v6/printers.py (access_streambuf_ptrs):
	New helper function.
	(StdStringBufPrinter, StdStringStreamPrinter): New printers.
	(build_libstdcxx_dictionary): Register stringstream printers.
	* testsuite/libstdc++-prettyprinters/debug.cc: Check string
	streams.
	* testsuite/libstdc++-prettyprinters/simple.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/simple11.cc: Likewise.