[MLIR] Remove dependency on omp dialect in LLVM dialect.

This fixes a buildbot failure where the dependency on the omp dialect
in the LLVM dialect caused error. Instead of accessing the interface
defined in the omp dialect we directly access the attributes
instead. To make this work the IsDeviceAttr is removed and replaced
with a BoolAttr instead.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D151745