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
This fixes sudden compiler errors that are emitted when trying to use CoreText.framework with C++ interop enabled.
When Swift is trying to rebuild a dependency module from its textual interface, it should not complain on usages of C enums in public Swift APIs.
This still leaves the resilience safety guardrail enabled for C++ scoped enums.
rdar://143215914
Changing the members of these structs are still API and ABI breaking
changes but they are not as fragile as C++ classes where adding/removing
virtual functions or doing other changes can also result in breaking the
ABI.
rdar://119319825
