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

[via llvm-commits]

zmodem wrote:

Sounds great to me. Getting this into the windows packages without having to jump through hoops will be very nice (and I look forward to removing the hoop jumping we do in Chromium's clang build script).

I assume this is picking up the latest rpmalloc release (1.4.5)? We should document somewhere what exact version is being used.

Is there a plan for keeping llvm's rpmalloc copy up-to-date?

Do we want to keep the general `LLVM_INTEGRATED_CRT_ALLOC` support going forward?

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