Merge commit 18efa420da5f from llvm git (by Brooks Davis)
compiler-rt: Allow build without __c11_atomic_fetch_nand
Don't build atomic fetch nand libcall functions when the required
compiler builtin isn't available. Without this compiler-rt can't be
built with LLVM 13 or earlier.
Not building the libcall functions isn't optimal, but aligns with the
usecase in FreeBSD where compiler-rt from LLVM 14 is built with an LLVM
13 clang and no LLVM 14 clang is built.
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D126710
MFC after: 3 days
cfefd16d57