normative: allow await in parameter defaults (#917)

[pemrouz]

This addresses a spec bug to allow using the await keyword in parameter
defaults for non-arrow async functions:

async function foo(bar = await baz) { ... }

This enables developers to have a consistent way to default parameters
using both async and non-async values, which otherwise quickly becomes
a common pain point. A complete motivating example is described in [1].

The solution converged on here after several iterations has been refined
to avoid creating unnecessary duplication between the different cases. In
EvaluateBody (14.7.11) the promise is already created before
FunctionDeclarationInstantiation (which is also already rejected if
initialisation throws). Instead of creating an
AsyncFunctionDeclarationInstantiation, creating additional promises, or
duplicating the boilerplate in AsyncFunctionStart, the call to FDI has
simply been moved into AsyncFunctionStart which then allows usage of the
await keyword during initialisation. It also removes the corresponding
early errors in 14.7.1 (Static Semantics: Early Errors) and 19.2.1.1.1
(CreateDynamicFunction).

This PR is to use a basis for discussion in the committee. We are already
aware of and plan to make the analogous change to async generators before
landing.

Reviewers: @littledan, @bterlson, @syg
Tests:
https://github.com/tc39/test262/compare/master...pemrouz:normative/await-in-parameter-defaults?expand=1

[1] #917 (comment)

[littledan]

The plan is to extend this change to async generators, right?

[bakkot]

I'm going to close this in light of #917 (comment) and the discussion in plenary.