Adds `transform.func.cast_and_call` that takes a set of inputs and
outputs and replaces the uses of those outputs with a call to a function
at a specified insertion point.
The idea with this operation is to allow users to author independent IR
outside of a to-be-compiled module, and then match and replace a slice
of the program with a call to the external function.
Additionally adds a mechanism for populating a type converter with a set
of conversion materialization functions that allow insertion of
casts on the inputs/outputs to and from the types of the function
signature.
This revision adds a `transform.apply_conversion_patterns.func.func_to_llvm` transformation.
It is unclear at this point whether this should be spelled out as a standalone transformation
or whether it should resemble `transform.apply_conversion_patterns.dialect_to_llvm "fun"`.
This is dependent on how we want to handle the type converter creation.
In particular the current implementation exhibits the fact that
`transform.apply_conversion_patterns.memref.memref_to_llvm_type_converter` was not rich enough
and did not match the LowerToLLVMOptions.
Keeping those options in sync across all the passes that lower to LLVM is very error prone.
Instead, we should have a single `to_llvm_type_converter`.
Differential Revision: https://reviews.llvm.org/D157553
