[lld] [ELF] PHDRS update while condition and phdrs.s unclose2.lds output (PR #100918)

[Hongyu Chen via llvm-commits]

yugier wrote:

> Using `till(";")` for the inner loop would cause inferior diagnostics for the following case:
> 
> ```
> ld.lld: error: 1.lds:1: unexpected header attribute: }
> >>> PHDRS {all PT_LOAD FILEHDR PHDRS }
> >>> 
> ```
> 
> something like `expected ';'` would be better

I was trying to figure out a better way for this but its difficult to have a clean code unless we have more states to help us to identify if current token is an identifier/operator/symbol. 

```
void ScriptParser::readPhdrs() {
  expect("{");
  while (auto tok = till("}")) {
    PhdrsCommand cmd;
    cmd.name = tok;
    cmd.type = readPhdrType();

    while (!atEOF()) {
      if (consume("FILEHDR"))
        cmd.hasFilehdr = true;
      else if (consume("PHDRS"))
        cmd.hasPhdrs = true;
      else if (consume("AT"))
        cmd.lmaExpr = readParenExpr();
      else if (consume("FLAGS"))
        cmd.flags = readParenExpr()().getValue();
      else
        break;
        //setError("unexpected header attribute: " + next());
    }
    expect(";");
    script->phdrsCommands.push_back(cmd);
  }
}
```
Without a clear state the code above is the one i can think about the most concise one but we need to modify the error message. 

https://github.com/llvm/llvm-project/pull/100918