ASTContext: Factor 'getObjCEncodingForPropertyType' as its own method.

It is useful to get the property encoding for an ObjC type without a full
ObjCPropertyDecl.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212496 91177308-0d34-0410-b5e6-96231b3b80d8