Add LIBC_ASSERT statements to FixedVector implementation, and zero out
the memory when the elements are removed to flag out-of-bound access and
dangling pointer/reference access.

This change unmasks the bug in one of FixedVector uses for atexit
handlers: dangling reference use, which was actually led to crashes in
the wild (with prod blockstore implementation). Fix it in this CL.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This refactors some of the FreeListHeap, FreeList, and Block classes to
have constexpr ctors so we can constinit a global allocator that does
not require running some global function or global ctor to initialize.
This is needed to prevent worrying about initialization order and any
other module-ctor can invoke malloc without worry.

This adds:
- A ctor accepting a start and end iterator
- A ctor accepting a count and const T&
- size()
- subscript operators
- begin() and end() iterators

- Implements forward iterators for `cpp::fixed_vector` to use in
https://github.com/llvm/llvm-project/pull/92009

Critically, we don't want to return an iterator to the end of the underlying
cpp::array "store." Add a test to catch this issue.

This will be used by __cxa_finalize to iterate backwards through a FixedVector.

Link: #85651