Add a __builtin_addressof that performs the same functionality as the built-in

Add a __builtin_addressof that performs the same functionality as the built-in
& operator (ignoring any overloaded operator& for the type). The purpose of
this builtin is for use in std::addressof, to allow it to be made constexpr;
the existing implementation technique (reinterpret_cast to some reference type,
take address, reinterpert_cast back) does not permit this because
reinterpret_cast between reference types is not permitted in a constant
expression in C++11 onwards.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186053 91177308-0d34-0410-b5e6-96231b3b80d8