[mlir] StandardToLLVM: make one-to-one convresion pattern publicly available

Summary:
The Standard-to-LLVM dialect convresion has a set of utility classes that
simplify conversions, including patterns that provide one-to-one conversion
operation conversion with optional result packing. Expose these classes in a
public header so that conversions other than Standard-to-LLVM (e.g. vectors, or
LLVM-based intrinsics) could also use them. Since the patterns are implemented
as class templates and in order to keep the code size limited, keep the
implementation private by resorting to op identifiers instead of template-based
builders.

Differential Revision: https://reviews.llvm.org/D76864