Remove lit feature "reverse_iteration"

[MaskRay]

I added a lit feature reverse_iteration to work around two tests when implementing LLVM_ENABLE_REVERSE_ITERATION for StringMap and changing StringMap used hash function from xxHash64 to xxh3_64bits. We can remove the lit feature once the following 2 tests are fixed:

% rg ':.*reverse_iteration'
llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s
1:# REQUIRES: reverse_iteration

llvm/test/CodeGen/MIR/AMDGPU/virtreg-uses-unallocatable-class.mir
1:# UNSUPPORTED: reverse_iteration

@jayfoad @lhames

[lhames]

I haven't had a chance to check the RuntimeDyld failure yet, and probably won't for a while.

Unless this is high priority I'm inclined to let it die off with RuntimeDyld -- I'm hoping that we can announce deprecation of it (and MCJIT) at the dev meeting this year, and maybe remove it next year or the year after.

[MaskRay]

Thank you for the response! You may consider removing llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s then, before the code becomes more dead...