[coroutines] Fix assertion in DependentCoawaitExpr when the argument is non-dependent.

Summary:
A `co_await arg` expression has a dependent type whenever the promise type is still dependent, even if the argument to co_await is not. This is because we cannot attempt the `await_transform(<arg>)` until after we know the promise type.

This patch fixes an assertion in the constructor of `DependentCoawaitExpr` that asserted that `arg` must also be dependent.

Reviewers: rsmith, GorNishanov, aaron.ballman

Reviewed By: GorNishanov

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D30772

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297358 91177308-0d34-0410-b5e6-96231b3b80d8