PR11851 (and duplicates): Whenever a constexpr function is referenced,

PR11851 (and duplicates): Whenever a constexpr function is referenced,
instantiate it if it can be instantiated and implicitly define it if it can be
implicitly defined. This matches g++'s approach. Remove some cases from
SemaOverload which were marking functions as referenced when just planning how
overload resolution would proceed; such cases are not actually references.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167514 91177308-0d34-0410-b5e6-96231b3b80d8