[llvm] [Support] Vendor rpmalloc in-tree and use it for the Windows 64-bit release (PR #91862)

[Alexandre Ganea via llvm-commits]

aganea wrote:

> I assume this is picking up the latest rpmalloc release (1.4.5)? We should document somewhere what exact version is being used.
I've upgraded to 1.4.5 and added the version number information in README.md

> Is there a plan for keeping llvm's rpmalloc copy up-to-date?
I would assume after the initial commit, upgrades would be as regular contributions?

> Do we want to keep the general `LLVM_INTEGRATED_CRT_ALLOC` support going forward?
I would keep it for now for testing other allocators. I still find mimalloc being a very good contender and I hope it'll be eventually integrated into Windows through some application manifest. It'd be good too if someone with SCUDO experience would make it work `LLVM_INTEGRATED_CRT_ALLOC`.

https://github.com/llvm/llvm-project/pull/91862