decoder: Add support for LiteralValue as Constraint

[radeksimko]

Part of hashicorp/terraform-ls#496

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TODO

[radeksimko]

LGTM

[radeksimko]

Nitpick: This would not account for completion inside a (valid) multi-line expression, which is one of the reasons we use slightly more sophisticated solution for such an edge case, e.g. in Map

hcl-lang/decoder/expr_map_completion.go

Lines 143 to 148 in d469d16

	// check any incomplete configuration up to a terminating character
	fileBytes := m.pathCtx.Files[eType.Range().Filename].Bytes
	recoveredBytes := recoverLeftBytes(fileBytes, pos, func(offset int, r rune) bool {
	return isObjectItemTerminatingRune(r) && offset > recoveryPos.Byte
	})
	trimmedBytes := bytes.TrimRight(recoveredBytes, " \t")

Given that we only get to deal with primitive types here (which are mostly single-line), it should be fine to ignore. Multiline strings do exist (HEREDOC syntax) but we don't really use them in Terraform as constraint anywhere.

Implementing a perfect solution would be unnecessarily costly as we'd have to be looking for different terminating character or even a sequence of characters, depending on what type we can "guess".

TL;DR it's fine as is.

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Side note - I am surprised that config such as this

attr = "foo

is parsed by HCL at all and returns expression with range that we can take advantage of here, because a lot of other invalid configuration variations end up missing from the AST entirely (including the attribute), which is also what motivated that recovery logic linked above. 😅