Add a backend option to warn on a given stack size limit.

Add a backend option to warn on a given stack size limit.
Option: -mllvm -warn-stack-size=<limit>
Output (if limit is exceeded):
warning: Stack size limit exceeded (<actual size>) in <functionName>.

The longer term plan is to hook that to a clang warning.
PR:4072
<rdar://problem/13987214>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183552 91177308-0d34-0410-b5e6-96231b3b80d8