[mlir] support recursive types in type conversion infra

MLIR supports recursive types but they could not be handled by the conversion
infrastructure directly as it would result in infinite recursion in
`convertType` for elemental types. Support this case by keeping the "call
stack" of nested type conversions in the TypeConverter class and by passing it
as an optional argument to the individual conversion callback. The callback can
then check if a specific type is present on the stack more than once to detect
and handle the recursive case.

This approach is preferred to the alternative approach of having a separate
callback dedicated to handling only the recursive case as the latter was
observed to introduce ~3% time overhead on a 50MB IR file even if it did not
contain recursive types.

This approach is also preferred to keeping a local stack in type converters
that need to handle recursive types as that would compose poorly in case of
out-of-tree or cross-project extensions.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D113579

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