Fix namespace for MLIR Async Runtime

The MLIR Async runtime uses different namespacing for the header file,
and the definitions of its C API. The header file places the extern "C"
functions inside namespace mlir::runtime, and the definitions are not
in a namespace. This causes issues in cl.exe. It treats the declaration
and definition as different, and thus does not apply dllexport to the
definition, which leads to the mlir_async_runtime.dll containing no
definitions, and the mlir_async_runtime.lib not being generated.

This patch moves the namespace to cover the definitions, and thus
generates the dll correctly on Windows with cl.exe.

This was tested with Visual Studio C++ 19.28.29336.

Differential Revision: https://reviews.llvm.org/D95386