[libcxx] [test] Fix Clang -Wunused-local-typedef, part 3/3.

test/std/strings/string.classes/typedefs.pass.cpp
Actually test what basic_string's typedefs stand for.

test/std/utilities/meta/meta.trans/meta.trans.other/result_of11.pass.cpp
NotDerived and ND were completely unused.

test/std/utilities/utility/pairs/pairs.pair/default.pass.cpp
P2 was mistakenly not being used. Yes, that's
right: -Wunused-local-typedef CAUGHT A MISTAKE! AMAZING!

Fixes D29137.

llvm-svn: 294156