Record function attribute "stackrealign" instead of using backend option

-force-align-stack.

Also, make changes to the driver so that -mno-stack-realign is no longer
an option exposed to the end-user that disallows stack realignment in
the backend.

Differential Revision: http://reviews.llvm.org/D11815


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247451 91177308-0d34-0410-b5e6-96231b3b80d8