[flang] Fix pimpl idiom for IntrinsicProcTable.

The class IntrinsicProcTable uses the pimpl idiom and manages its own pointer-to-implementation.  However, it violates the rule-of-five and does not implement a move-constructor or assignment-operator. Due to differences between compilers in implementation copy elision, these may or may not be used. Due to the missing user implementation for resource handling, using the results in runtime errors.

Fix my using `std::unique_ptr` instead of custom resource management.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D88794