Make memref promotion during std->LLVM lowering the default calling convention

During the conversion from the standard dialect to the LLVM dialect,
memref-typed arguments are promoted from registers to memory and passed into
functions by pointer. This had been introduced into the lowering to work around
the abesnce of calling convention modeling in MLIR to enable better
interoperability with LLVM IR generated from C, and has been exerciced for
several months. Make this promotion the default calling covention when
converting to the LLVM dialect. This adds the documentation, simplifies the
code and makes the conversion consistent across function operations and
function types used in other places, e.g. in high-order functions or
attributes, which would not follow the same rule previously.

PiperOrigin-RevId: 285751280