This is necessary because we need to model its stack-allocation
behavior, although I'm not yet doing that in this patch because
StackNesting first needs to be taught to not try to move the
deallocation.
I'm not convinced that `async let` *should* be doing a stack allocation,
but it undoubtedly *is* doing a stack allocation, and until we have an
alternative to that, we will need to model it properly.
When creating an ExtendedASTNodeLoc from a SILLocation, if the
SILLocation passed in belongs to a swift::Stmt, we only ever use the
Stmt's StartLoc for the SourceLocation. If the SILLocation passed in,
has a SourceLocation that matches the EndLoc of the Stmt, we should
correctly set the primary ASTNodeTy PointerUnion's integer to 1, to
denote that the SourceLocation dervied from the Stmt points to the
EndLoc.
