[mlir] recursively convert builtin types to LLVM when possible

Given that LLVM dialect types may now optionally contain types from other
dialects, which itself is motivated by dialect interoperability and progressive
lowering, the conversion should no longer assume that the outermost LLVM
dialect type can be left as is. Instead, it should inspect the types it
contains and attempt to convert them to the LLVM dialect. Introduce this
capability for LLVM array, pointer and structure types. Only literal structures
are currently supported as handling identified structures requires the
converison infrastructure to have a mechanism for avoiding infite recursion in
case of recursive types.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D112550