[mlir] Add stack alignment to the data layout dialect.

The revision adds the stack alignment to the
data layout dialect and it extends the LLVM dialect
import and export to support the new data layout
entry.

One possible use case for the flag is the LLVM dialect
inliner. The LLVM inliner queries the flag to
determine if it is safe to update the alignment of an
existing alloca. We may want to perform the same
optimization inside of MLIR.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D147332