It was originally placed in TransformInterfaces for convenience, but it
is really a generic utility. It may also create an include cycle between
TransformTypes and TransformInterfaces if the latter needs to include
the former because the former uses the failure util.
Reviewed By: springerm
Differential Revision: https://reviews.llvm.org/D140978
This customer parser/printer is similar to DynamicIndexList, but has special syntax for the case where one handle represents the entire list.
Example:
```
// Regular index list
[10, 20, %val]
// Packed handle (no square parentheses)
%val
```
Differential Revision: https://reviews.llvm.org/D138825