[libstdc++] [prettyprint] add local std::string use to more tests

[libstdc++] [prettyprint] add local std::string use to more tests

We're getting regressions after ugprading to GDB 13 in a few of the
libstdc++ pretty-printing tests.  It might seem like a GDB regression,
but that update has a new symbol reader, and Tom Tromey says it
exposes a latent problem in the pretty-printer, namely, when a name is
not defined locally in the local translation unit, GDB will look for
it in other units, and if multiple different definitions are
available, there are no guarantees as to which one it will find.

Since libstdc++ contains units in which std::string maps to different
types, in order to ensure the tests get the std::string definition we
expect, we need std::string to be present in the debug information for
the test itself, as we already do in other tests.

for  libstdc++-v3/ChangeLog

	* testsuite/libstdc++-prettyprinters/80276.cc: Add
	std::string to debug info.
	* testsuite/libstdc++-prettyprinters/libfundts.cc: Likewise.