Add OMP exception capture helpers and migrate IndexIVF (facebookresearch#5111)

Summary:

Add utility functions to FaissException.h for capturing and rethrowing exceptions across OpenMP parallel region boundaries, and migrate IndexIVF to use them.

New helpers in FaissException.h:

- omp_capture_exception(ex): Captures the current exception into a shared exception_ptr using #pragma omp critical. Only records the first exception.
- omp_capture_exception(ex, cleanup): Overload with a per-thread cleanup callable (e.g. setting an interrupt flag) that runs inside the critical section.
- omp_rethrow_if_exception(ex): Rethrows the captured exception on the main thread after the parallel region completes.

The exception_ptr approach preserves the full exception during capture and preserves the original exception type on rethrow.

IndexIVF migration (search, search_preassigned, range_search_preassigned):

- Replace std::mutex + std::string exception_string with std::exception_ptr + omp_capture_exception/omp_rethrow_if_exception.
- Replace bool interrupt with std::atomic<bool> (the previous non-atomic bool was read without synchronization in the omp for loop while being written inside a mutex-guarded catch block — undefined behavior). Use relaxed atomic loads for the interrupt short-circuit check since there is no way to fully close the race between interrupt being set and a thread being in the middle of computation.
- Replace catch (const std::exception&) with catch (...) to capture all exception types.
- Original exception type is now preserved on rethrow instead of being reformatted into a FaissException wrapping demangle_cpp_symbol + what().
- Replace InterruptCallback::is_interrupted() + manual flag set with InterruptCallback::check(). check() throws on interrupt, which is captured by omp_capture_exception and sets the interrupt flag via the cleanup lambda. This eliminates the separate post-region interrupt check — all error paths now flow through the unified exception capture/rethrow mechanism.
- range_search_preassigned has no interrupt short-circuit check in its loops, so it uses omp_capture_exception(ex) without a cleanup lambda and has no interrupt variable.
- Wrap all #pragma omp for loop bodies in try/catch blocks so exceptions are caught within the worksharing construct (required by the OpenMP specification). The outer try/catch at the #pragma omp parallel level remains as a safety net for code outside worksharing constructs.
- Remove redundant try/catch from scan_one_list and scan_list_func lambdas since all call sites are now protected by the loop body try/catch.

Differential Revision: D101233059

Author: scsiguy