[mlir][flang] Convert TBAA metadata to an attribute representation

The current representation of TBAA is the very last in-tree user of the `llvm.metadata` operation.
Using ops to model metadata has a few disadvantages:
* Building a graph has to be done through some weakly typed indirection mechanism such as `SymbolRefAttr`
* Creating the metadata has to be done through a builder within a metadata op.
* It is not multithreading safe as operation insertion into the same block is not thread-safe

This patch therefore converts TBAA metadata into an attribute representation, in a similar manner as it has been done for alias groups and access groups in previous patches.

This additionally has the large benefit of giving us more "correctness by construction" as it makes things like cycles in a TBAA graph, or references to an incorrectly typed metadata node impossible.

Differential Revision: https://reviews.llvm.org/D155444