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

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

I think that the legal aspects have been resolved right? At that point, we should consider getting this into the tree so that others can easily play with it. I know that I am interested in exploring this as an alternative memory allocator for the Swift compiler as well.

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