When a function argument is annotated with the `llvm.byval` attribute,
[LLVM expects](https://llvm.org/docs/LangRef.html#parameter-attributes)
the function argument type to be an `llvm.ptr`. For example:

```
func.func (%args0 : llvm.ptr {llvm.byval = !llvm.struct<(i32)>} {
  ...
}
```

Unfortunately, this makes the type conversion context-dependent, which
is something that the type conversion infrastructure (i.e.,
`LLVMTypeConverter` in this particular case) doesn't support. For
example, we may want to convert `MyType` to `llvm.struct<(i32)>` in
general, but to an `llvm.ptr` type only when it's a function argument
passed by value.

To fix this problem, this PR changes the FuncToLLVM conversion logic to
generate an `llvm.ptr` when the function argument has a `llvm.byval`
attribute. An `llvm.load` is inserted into the function to retrieve the
value expected by the argument users.

In today's repo, attribute `llvm.emit_c_interface` of func op is handled
outside of `mlir::convertFuncOpToLLVMFuncOp` in `FuncOpConversion`
pattern. In some cases, `FuncOpConversion` can't be directly re-used,
but we still want to re-use the code to emit c interface for
`llvm.emit_c_interface`.

Changes in this PR
* move the code to generate c with "llvm.emit_c_interface" interface
into `mlir::convertFuncOpToLLVMFuncOp` to be able to re-use it.
* added unit test to verify c interface for jit can be generated
correctly if only call `convertFuncOpToLLVMFuncOp`.
* removed `FuncOpConversionBase`

---------

Co-authored-by: Fung Xie <ftse@nvidia.com>