[stackprotector] Refactor out the end of isInTailCallPosition into the function returnTypeIsEligibleForTailCall.

This allows me to use returnTypeIsEligibleForTailCall in the stack protector pass.

rdar://13935163

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188765 91177308-0d34-0410-b5e6-96231b3b80d8
[stackprotector] Added support for emitting the llvm intrinsic stack protector check.

rdar://13935163

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188766 91177308-0d34-0410-b5e6-96231b3b80d8