hcldec: Improved context for expression diagnostics

[apparentlymart]

The two cases where we decode attribute values should include the expression and EvalContext in any diagnostics they generate so that the calling application can give hints about the types and values of variables that are used within the expression.

This also includes some adjustments to the returned source ranges so that both cases are consistent with one another and so that both indicate the entire expression as the Subject and include the attribute name in the Context. Including the whole expression in the range ensures that when there's a problem inside a tuple or object constructor, for example, we'll show the line containing the problem and not just the opening [ or { symbol.

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We don't have any specific tests in HCL for the metadata on diagnostics so in order to test this I linked it into Terraform and did the mistake that most commonly causes this error in Terraform usage:

Error: Incorrect attribute value type

  on wrong-type-context-msg.tf line 11, in resource "null_resource" "foo":
  11:   triggers = {
  12:     foo = "woooo"
  13:     bar = local.thingy
  14:   }
    |----------------
    | local.thingy is tuple with 2 elements

Inappropriate value for attribute "triggers": element "bar": string required.

Before this change, the message looked like this instead:

Error: Incorrect attribute value type

  on wrong-type-context-msg.tf line 11, in resource "null_resource" "foo":
  11:   triggers = {

Inappropriate value for attribute "triggers": element "bar": string required.

[mildwonkey]

Looks great!