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

[arichardson]

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;
  }

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: Alexander Richardson (arichardson)

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;
  }