[llvm] Implement reserveAllocationSpace for SectionMemoryManager (PR #71968)

[Anthonin Bonnefoy via llvm-commits]

bonnefoa wrote:

To add an additional impacted project, I had this issue happening on PostgreSQL instances. I've written more details about the impact in a [message](https://www.postgresql.org/message-id/flat/CAO6_Xqr63qj%3DSx7HY6ZiiQ6R_JbX%2B-p6sTPwDYwTWZjUmjsYBg%40mail.gmail.com) to the pgsql-hackers mailing list.

I've also tested the patched SectionMemoryManager on an impacted database and it fixed the segfaults.

https://github.com/llvm/llvm-project/pull/71968