CppEditor: Fix insert position in MoveFuncDefToDecl

When a class was directly assigned to a variable the definition was
misplaced right after the variable.

Task-number: QTCREATORBUG-10303
Change-Id: I2cdfee784b085d856d7ff5ebe62bf791b9a6754e
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>