[mlir] do not require LLVMDialect in conversion from LLVM IR

Historically, LLVMDialect has been required in the conversion from LLVM IR in
order to be able to construct types. This is no longer necessary with the new
type model and the dialect can be replaced with a local LLVM context.

Reviewed By: rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D85444