Promote nameless lambda used by dl_iterate_phdr to named function to clean up...

Promote nameless lambda used by dl_iterate_phdr to named function to clean up control flow inside findUnwindSections. Also, expose the data structure
to allow use by a future replacment function.

Summary: [Refactor] Promote nameless lambda to fully named function, allowing easy replacement in following patch.

Subscribers: krytarowski, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75480