[mlir] Prevent implicit downcasting to interfaces

Currently conversions to interfaces may happen implicitly (e.g.
`Attribute -> TypedAttr`), failing a runtime assert if the interface
isn't actually implemented. This change marks the `Interface(ValueT)`
constructor as explicit so that a cast is required.

Where it was straightforward to I adjusted code to not require casts,
otherwise I just made them explicit.

Depends on D148491, D148492

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D148493