[mlir][llvm] Support importing magic globals

[mlir][llvm] Support importing magic globals

This commit adds support for importing the magic globals "global_ctors"
and "global_dtors" from LLVM IR to the LLVM IR dialect. The import
fails when these globals have a non-null data pointer, as this can
currently not be represented in the corresponding MLIR operations.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D140877