Wdeprecated: LambdaScopeInfos are copied in TreeTransform, so make sure they're copyable.

Partly addressed by r244843, but the explicit dtor in LambdaScopeInfo
was still thwarting the implicit copy ctor. This does remove the key
function from LambdaScopeInfo unfortunately, but it seems neater than
having to explicitly default any special members LambdaScopeInfo needs.

llvm-svn: 244957