Make DiagnosticsEngine non-copyable

Also provide an out-of-line dtor for CompilerInvocation.

Cleanup work that may help reduce header inclusion for IntrusiveRefCntPtr.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208512 91177308-0d34-0410-b5e6-96231b3b80d8