[libc++] Refactor .compile.fail.cpp tests for std::function

Some of those .compile.fail.cpp tests had become incorrect and they
were not testing anything. In general, .compile.fail.cpp tests are
bad because they make it way too easy to write garbage tests. Indeed,
the test could fail to compile due to any reason whatsoever (even a
typo) and it would appear to work correctly.

Differential Revision: https://reviews.llvm.org/D138731