[objc-arc] Performed some small cleanups in ARCRuntimeEntryPoints and added an llvm_unreachable after the switch to quiet -Wreturn_type errors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185746 91177308-0d34-0410-b5e6-96231b3b80d8
isKnownToBeAPowerOfTwo: Fix a typo in a comment


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185748 91177308-0d34-0410-b5e6-96231b3b80d8
[llvm-ar] Added llvm_unreachable to quiet -Wreturn-type warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185751 91177308-0d34-0410-b5e6-96231b3b80d8