clone: managed boxes need to clone by shallow copy

Performing a deep copy isn't ever desired for a persistent data
structure, and it requires a more complex implementation to do
correctly. A deep copy needs to check for cycles to avoid an infinite
loop.
auto merge of #5763 : thestinger/rust/clone, r=nikomatsakis

Performing a deep copy isn't ever desired for a persistent data
structure, and it requires a more complex implementation to do
correctly. A deep copy needs to check for cycles to avoid an infinite
loop.
Update LLVM for ARM changes
rustllvm: followup latest LLVM
rust morestack assembly for arm

Conflicts:
	src/rt/arch/arm/morestack.S
FIX meta_section_name for macos
rustllvm: Initialize target analysis passes

Without this the target info for certain optimizations will not be
created and the compiler will sometimes crash
add unwind information on morestack