Remove the `conversionCallStack` from the MLIR TypeConverter

This vector keeps tracks of recursive types through the recursive invocations
of `convertType()`. However this is something only useful for some specific
cases, in which the dedicated conversion callbacks can handle this stack
privately.

This allows removing a mutable member of the type converter.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158351