[mlir] Placeholder used in predicate should be the base type

Added a notification in the placeholder section. While writing things
like preciate of an attribute, we may embed certain placeholder in the C
expression. Note that the type of the placeholder is only guaranteed to
be the base type like mlir::Type, it's better not to use the derived
type which is based on the implementation.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D112396