[llvm] [TableGen] Add let append/prepend syntax for field concatenation (PR #182382)

[Henrich Lauko via llvm-commits]

xlauko wrote:

> Nice. This is something I've wanted before. Does it also work with nesting top-level `let appending X in {`? Maybe that needs a testcase? This would help for nesting `Predicates` definitions for instructions/patterns

@lenary Do you mean something like:
```
class Base {
  list<int> items = [1, 2];
}

// Test nested top-level let append.
let append items = [100] in {
  let append items = [200] in {
    // CHECK: def NestedTopLevel
    // CHECK: list<int> items = [1, 2, 100, 200];
    def NestedTopLevel : Base;
  }
}
```

This already works and is tested, though has, at least for my taste, a bit weird semantics dut to TableGen execution, so outer most append is applied first.

https://github.com/llvm/llvm-project/pull/182382