[Stack Alignment] emit warning instead of a hard error

Per discussion in rdar://13127907, we should emit a hard error only if
people write code where the requested alignment is larger than achievable
and assumes the low bits are zeros. A warning should be good enough when
we are not sure if the source code assumes the low bits are zeros.

rdar://13127907


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174336 91177308-0d34-0410-b5e6-96231b3b80d8