Once the '@escaping' bit is removed from TupleTypeElt, it no longer makes
sense to print argument lists as if they were TupleTypes or ParenTypes,
since function types are '@escaping' by default inside tuples but not
in argument lists.
Instead, print ArrayRef<AnyFunctionType::Param> directly. For now this
introduces some awkward usages of AnyFunctionType::decomposeInput();
these will go away once the AST is changed to represent the argument list
as a list of expressions and not a single tuple expression.
Merge logic from `diagnoseAssignmentFailure` and `diagnoseSubElementFailure`
into new `AssignmentFailure`, together with their support functions, which
decouples `CSDiagnostics` from `CSDiag` and scrubs latter from some functionality.
