[Sema] Delete unused declarations

Notes about some declarations:

* clang::Sema::endsWithnarrowing: deleted by rC148381
* clang::Sema::ConvertIntegerToTypeWarnOnOverflow: deleted by rC214678
* clang::Sema::FreePackedContext: deleted by rC268085
* clang::Sema::ComputeDefaulted*: deleted by rC296067