Initial version of the LLVM IR dialect

LLVM IR types are defined using MLIR's extendable type system.  The dialect
provides the only type kind, LLVMType, that wraps an llvm::Type*.  Since LLVM
IR types are pointer-unique, MLIR type systems relies on those pointers to
perform its own type unique'ing.  Type parsing and printing is delegated to
LLVM libraries.

Define MLIR operations for the LLVM IR instructions currently used by the
translation to the LLVM IR Target to simplify eventual transition.  Operations
classes are defined using TableGen.  LLVM IR instruction operands that are only
allowed to take constant values are accepted as attributes instead.  All
operations are using verbose form for printing and parsing.

PiperOrigin-RevId: 229400375