[Clang-Format] Tablegen formatter should prefer breaking after `let ... in`

We currently end up with formatting like the following (running clang-format on RISCVInstrInfo.td:
```
let isTrap =
1 in def EBREAK : RVInstI<0b000, OPC_SYSTEM, (outs), (ins), "ebreak", "">,
Sched<[]> {
let rs1 = 0;
let rd = 0;
let imm12 = 1;
}
```

It looks better with curly braces, but still not ideal:
```
let isTrap = 1 in {
def EBREAK : RVInstI<0b000, OPC_SYSTEM, (outs), (ins), "ebreak", "">,
Sched<[]> {
let rs1 = 0;
let rd = 0;
let imm12 = 1;
}
}
```

IMO it would be cleanest to use something like this, i.e. break after `let ... in` and no indentation.
```
let isTrap = 1 in
def EBREAK : RVInstI<0b000, OPC_SYSTEM, (outs), (ins), "ebreak", "">,
Sched<[]> {
let rs1 = 0;
let rd = 0;
let imm12 = 1;
}
```

Reproduce the formatting in RISCVInstrInfo.td by running clang-format and compare the current output with the proposed break after `let ... in`. Trace the TableGen formatter behavior responsible for this construct, then verify that the resulting output matches the example without regressing related formatting.

Written by the indexing model from the issue text.