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

[via llvm-commits]

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@@ -62,6 +62,9 @@ Changes to LLVM infrastructure
 Changes to building LLVM
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+* LLVM now supports rpmalloc in-tree, as a replacement C allocator for hosted
+  toolchains. Currently using version 1.4.5.
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zmodem wrote:

Maybe this could be expanded a bit, with a link to the relevant section in CMake.rst, mentioning that the relevant flag is LLVM_ENABLE_RPMALLOC, that the main benefit is for LTO on Windows etc?

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