[mlir][EmitC] Add func, call and return operations and conversions (#79612)

This adds a `func`, `call` and `return` operation to the EmitC dialect,
closely related to the corresponding operations of the Func dialect. In
contrast to the operations of the Func dialect, the EmitC operations do
not support multiple results. The `emitc.func` op features a
`specifiers` argument that for example allows, with corresponding
support in the emitter, to emit `inline static` functions.

Furthermore, this adds patterns and a pass to convert the Func dialect
to EmitC. A `func.func` op that is `private` is converted to
`emitc.func` with a `"static"` specifier.