Fix for ambiguous lookup in expressions between local variable and namespace

Summary:
In an Objective-C context a local variable and namespace can cause an ambiguous name lookup when used in an expression. The solution involves mimicking the existing C++ solution which is to add local using declarations for local variables. This causes a different type of lookup to be used which eliminates the namespace during acceptable results filtering.

Differential Revision: https://reviews.llvm.org/D59960

llvm-svn: 359921