[WPD]Regard unreachable function as a possible devirtualizable target (#115668)

https://reviews.llvm.org/D115492 skips unreachable functions and
potentially allows more static de-virtualizations. The motivation is to
ignore virtual deleting destructor of abstract class (e.g.,
`Base::~Base()` in https://gcc.godbolt.org/z/dWMsdT9Kz).
* Note WPD already handles most pure virtual functions (like `Base::x()`
in the godbolt example above), which becomes a `__cxa_pure_virtual` in
the vtable slot.

This PR proposes to undo the change, because it turns out there are
other unreachable functions that a general program wants to run and fail
intentionally, with `LOG(FATAL)` or `CHECK` [1] for example. While many
real-world applications are encouraged to check-fail sparingly, they are
allowed to do so on critical errors (e.g., misconfiguration or bug is
detected during server startup).
* Implementation-wise, this PR keeps the one-bit 'unreachable' state in
bitcode and updates WPD analysis.
 
https://gcc.godbolt.org/z/T1aMhczYr is a minimum reproducible example
extracted from unit test. `Base::func` is a one-liner of `LOG(FATAL) <<
"message"`, and lowered to one basic block ending with `unreachable`. A
real-world program is _allowed_ to invoke Base::func to terminate the
program as a way to report errors (in server initialization stage for
example), even if errors on the serving path should be handled more
gracefully.

[1] https://abseil.io/docs/cpp/guides/logging#CHECK and
https://abseil.io/docs/cpp/guides/logging#configuration-and-flags