Update Atom Silvermont (SLM) support by adding enabled features.



git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@190718 91177308-0d34-0410-b5e6-96231b3b80d8
Switching the WeakRef attribute to using the new checkStringLiteralArgument helper function.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@190719 91177308-0d34-0410-b5e6-96231b3b80d8