Fix MLIR Async Runtime DLL on Windows

The AsyncRuntime declares prototypes for extern "C" functions inside a
namespace in the header, but not inside that namespace in the
definition. This causes Visual Studio to treat them as different
entities and thus the dllexport is ignored for the definitions.

Using the same namespace fixes this issue.

Secondly, this commit moves the dllexport to be consistent with the
JITs expectation.

This is an update to https://reviews.llvm.org/D95386 that fixes the
compile issues in old versions of Visual studio.

Differential Revision: https://reviews.llvm.org/D95933