`Esm5ReflectionHost#getInnerFunctionDeclarationFromClassDeclaration()`
was already called with `ts.Declaration`, not `ts.Node`, so we can
tighten its parameter type and get rid of a redundant check.
`getIifeBody()` (called inside
`getInnerFunctionDeclarationFromClassDeclaration()`) will check whether
the given `ts.Declaration` is a `ts.VariableDeclaration`.
PR Close#35527