[ITA] llvm

[Jeremy Drake]

I just saw you updated some patches.  Most of the patches are now already
upstream and are included to backport to 20.1.x.
(https://github.com/msys2/MSYS2-packages/blob/master/llvm/README-patches.md)
Specifically, your modification to the clang 0110 patch adding a Cygwin
toolchain should be a followup patch because 0110 patch is
https://github.com/llvm/llvm-project/commit/52924a2d7255cdd280b2b82dad8616e01fe065da

I had been intending to get back to a follow-up PR to be able to call the
linker directly, but never did.  I was thinking about maybe trying to
derive from the MinGW linker...

Also, I am confused about #define-ing constinit in the libc++ patches.  Is
there still some issue with constinit?