[Support] On Windows, add optional support for {rpmalloc|snmalloc|mimalloc}

This patch optionally replaces the CRT allocator (i.e., malloc and free) with rpmalloc (mixed public domain licence/MIT licence) or snmalloc (MIT licence) or mimalloc (MIT licence). Please note that the source code for these allocators must be available outside of LLVM's tree.

To enable, use `cmake ... -DLLVM_INTEGRATED_CRT_ALLOC=D:/git/rpmalloc -DLLVM_USE_CRT_RELEASE=MT` where `D:/git/rpmalloc` has already been git clone'd from `https://github.com/mjansson/rpmalloc`. The same applies to snmalloc and mimalloc.

When enabled, the allocator will be embeded (statically linked) into the LLVM tools & libraries. This currently only works with the static CRT (/MT), although using the dynamic CRT (/MD) could potentially work as well in the future.

When enabled, this changes the memory stack from:
  new/delete -> MS VC++ CRT malloc/free -> HeapAlloc -> VirtualAlloc
to:
  new/delete -> {rpmalloc|snmalloc|mimalloc} -> VirtualAlloc

The goa...