Provide completion in unterminated ForExpr

[radeksimko]

Background

While implementing support for ForExpr in #368 a few edge cases surfaced around completion:

• incomplete ForExpr
  • attr = [ for k, v in /*HERE*/ ]
  • attr = [ for k, v in coll if /*HERE*/ ]
  • attr = { for k, v in /*HERE*/ }
  • attr = { for k, v in coll if /*HERE*/ }
• "impeding" incomplete Traversal (with trailing dot) inside otherwise complete ForExpr
  • attr = [ for k, v in var./*HERE*/ ]
  • attr = [ for k, v in coll if var./*HERE*/ ]
  • attr = { for k, v in var./*HERE*/ }
  • attr = { for k, v in coll if var./*HERE*/ }

In all of the above cases we're unable to provide any completion primarily because the upstream HCL parser is thrown off by the invalid stuff which causes it to return "empty literal value expression" such as the one below:

&hclsyntax.LiteralValueExpr{
    Val: cty.Value{
        ty: cty.Type{},
        v:  &cty.unknownType{},
    },
    SrcRange: hcl.Range{
        Filename: "test.hcl",
        Start:    hcl.Pos{Line:1, Column:8, Byte:7},
        End:      hcl.Pos{Line:1, Column:24, Byte:23},
    },
}

Proposal

Investigate either changes upstream in HCL or some form of recovery downstream here in hcl-lang that would enable completion in the above contexts.