CodeGen: ensure placeholder instruction for cleanup is created

A placeholder instruction for use in generation of cleanup code for an
initializer list would not be emitted if the base class contained a
non-trivial destructor and the class contains no fields of its own. This
would be the case when using CTAD to deduce the template arguments for a
struct with an overloaded call operator, e.g.

```
template <class... Ts> struct ctad : Ts... {};
template <class... Ts> ctad(Ts...)->ctad<Ts...>;
```

and this class was initialized with a list of lambdas capturing by copy,
e.g.

```
ctad c {[s](short){}, [s](long){}};
```

In a release build the bug would manifest itself as a crash in the SROA
pass, however, in a debug build the following assert in CGCleanup.cpp
would fail:

```
assert(dominatingIP && "no existing variable and no dominating IP!");
```

By ensuring that a placeholder instruction is emitted even if there's no
fields in the class, neither the assert nor the crash is reproducible.

See https://bugs.llvm.org/show_bug.cgi?id=40771

Patch by Øystein Dale!

llvm-svn: 367042