[coroutines] Don't build promise init with no args

[coroutines] Don't build promise init with no args

Summary:
In the case of a coroutine that takes no arguments,
`Sema::buildCoroutinePromise` constructs a list-initialization
(`clang::InitializationKind::InitKind::IK_DirectList`) of the
promise variable, using a list of empty arguments. So, if one were to
dump the promise `VarDecl` immediately after `Sema::ActOnCoroutineBodyStart`
calls `checkCoroutineContext`, for a coroutine function that takes no
arguments, they'd see the following:

```
VarDecl 0xb514490 <test.cpp:26:3> col:3 __promise '<dependent type>' callinit
`-ParenListExpr 0xb514510 <col:3> 'NULL TYPE'
```

But after this patch, the `ParenListExpr` is no longer constructed, and
the promise variable uses default initialization
(`clang::InitializationKind::InitKind::IK_Default`):

```
VarDecl 0x63100012dae0 <test.cpp:26:3> col:3 __promise '<dependent type>'
```

As far as I know, there's no case in which list-initialization with no
arguments differs from default initialization, but if I'm wrong please
let me know (and I'll add a test case that demonstrates the change --
but as-is I can't think of a functional test case for this). I think both
comply with the wording of C++20 `[dcl.fct.def.coroutine]p5`:

> _promise-constructor-arguments_ is determined as follows: overload
  resolution is performed on a promise constructor call created by
  assembling an argument list with lvalues `p1 ... pn`. If a viable
  constructor is found (12.4.2), then _promise-constructor-arguments_
  is `(p1, ... , pn)`, otherwise _promise-constructor-arguments_ is
  empty.

Still, I think this patch is an improvement regardless, because it
reduces the size of the AST.

Reviewers: GorNishanov, rsmith, lewissbaker

Subscribers: EricWF, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70555