Add emitc.tu

[mgehre-amd]

A emitc.tu represents a translation unit that can be emitted
into a single C++ file.

mlir-translate emits only the translation unit selected via
the -translation-unit-id=id flag. By default, no translation units are
emitted.

Example:

emitc.tu "main" {
  emitc.func @func_one() {
    emitc.return
  }
}

[josel-amd]

LGTM

[TinaAMD]

LGTM