https://bugs.llvm.org/show_bug.cgi?id=41909 describes an issue in which
a generic lambda that takes a dependent argument auto set causes the
template instantiation machinery for coroutine body statements to crash
with an ICE. The issue is two-fold:
- The paths taken by the template instantiator contain several asserts that the coroutine promise must not have a dependent type.
- The template instantiator unconditionally builds corotuine statements that depend on the promise type, which cannot be dependent.
To work around the issue, prevent the template instantiator from building
dependent coroutine statements if the coroutine promise type is dependent.
Since we only expect this to occur in the case of a generic lambda, limit
the workaround behavior to just that case.