Reapply r183552. This time, use a standard type for the option to avoid template
instantiation issue with non-standard type.

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>.