[mlir] Add support for LLVM's dso_local attr

This patch brings support for setting runtime preemption specifiers of
LLVM's GlobalValues. In LLVM semantics, if the `dso_local` attribute
is not explicitly requested, then it is inferred based on linkage and
visibility. We model this same behavior with a UnitAttribute: if it is
present, then we explicitly request the GlobalValue to marked as
`dso_local`, otherwise we rely on the GlobalValue itself to make this
decision.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D104983