[Sema] Allow casting of some overloaded functions

Some functions can't have their address taken. If we encounter an
overload set where only one of the candidates can have its address
taken, we should automatically select that candidate in cast
expressions.

Differential Revision: http://reviews.llvm.org/D17701


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263887 91177308-0d34-0410-b5e6-96231b3b80d8