[6.2.1][cxx-interop] Do not consider function types fragile
Explanationion: Function pointer types wee always considered fragile in C++ mode,
this manifested as a regression when interfacing with glibc.
Issues: rdar://159184118
Original PRs: #84040
Risk: Low, this only removes a spurious error for library evolution.
Testing: Added a compiler test.
Reviewers: @egorzhdan