Fixes hashicorp#737

When the `DiagnosticTextWriter` writes out a diagnostic,
if the diagnostic has an Expression and EvalContext,
it will also write out the evaluation context
in which the diagnostic occurred.

The `cty.Value` of the variable relevant to the expression
will be rendered as a string by `valueStr`, but this does not
take into account marked values and will cause a panic by
`val.AsString()`, `val.LengthInt'()`, etc.

This change prevents panics by always returning "(marked value)"
for marked values in `valueStr`. Whether this is always
appropriate is debatable, but it is better than
the current situation, which causes panics.

Fixes hashicorp#737

When the `DiagnosticTextWriter` writes out a diagnostic,
if the diagnostic has an Expression and EvalContext,
it will also write out the evaluation context
in which the diagnostic occurred.

The `cty.Value` of the variable relevant to the expression
will be rendered as a string by `valueStr`, but this does not
take into account marked values and will cause a panic by
`val.AsString()`, `val.LengthInt()`, etc.

This change prevents panics by always returning "(marked value)"
for marked values in `valueStr`. Whether this is always
appropriate is debatable, but it is better than
the current situation, which causes panics.

Fixes #2243
See also hashicorp/hcl#737

The `hcl.DiagnosticTextWriter` has an issue where it will panic
when writing out a diagnostic that has an expression containing
a marked value.

TFLint can run into this issue when expanding the `for_each`
meta-argument with a sensitive value. If the `for_each` is not
iterable (it is not iterable if marked), a diagnostic containing
the expression context of the `for_each` is created and written
to stderr by the `DiagnosticTextWriter`.

Hopefully this will be fixed in upstream, but here's a workaround
to prevent the panic by excluding expression context from the
diagnostic when the `for_each` is marked.

By the way, other cases that include expression context in
diagnostics do not have the same issue, so no action is required.

- `for_each` is marked in dynamic blocks
  - The value is always unmarked, and non-iterable cases are rarer than those in meta-arguments
- dynamic block labels
  - Dynamic block labels are rarely used in Terraform
- `count` in meta-arguments
  - The value is always unmarked

Fixes hashicorp#737
Follow up of hashicorp#738

When the `DiagnosticTextWriter` writes out a diagnostic,
if the diagnostic has an Expression and EvalContext,
it will also write out the evaluation context
in which the diagnostic occurred.

The `cty.Value` of the variable relevant to the expression
will be rendered as a string by `valueStr`, but calls to
`AsString` or `LengthInt` here will panic on marked values.

It is not clear how HCL should represent such marks,
so we skip the marked values from the additional context
as a best effort to be a general-purpose diagnostic writer,
and can consider how to output them later if needed.

Fixes hashicorp#737
Follow up of hashicorp#738

When the `DiagnosticTextWriter` writes out a diagnostic,
if the diagnostic has an Expression and EvalContext,
it will also write out the evaluation context
in which the diagnostic occurred.

The `cty.Value` of the variable relevant to the expression
will be rendered as a string by `valueStr`, but calls to
`AsString` or `LengthInt` here will panic on marked values.

It is not clear how HCL should represent such marks,
so we skip the marked values from the additional context
as a general-purpose diagnostic writer,
and can consider how to output them later if needed.

Fixes hashicorp#737
Follow up of hashicorp#738

When the `DiagnosticTextWriter` writes out a diagnostic,
if the diagnostic has an Expression and EvalContext,
it will also write out the evaluation context
in which the diagnostic occurred.

The `cty.Value` of the variable relevant to the expression
will be rendered as a string by `valueStr`, but calls to
`AsString` or `LengthInt` here will panic on marked values.

It is not clear how HCL should represent such marks,
so we skip the marked values from the additional context
as a general-purpose diagnostic writer.

Fixes #2243
See also hashicorp/hcl#737

The `hcl.DiagnosticTextWriter` has an issue where it will panic
when writing out a diagnostic that has an expression containing
a marked value.

TFLint can run into this issue when expanding the `for_each`
meta-argument with a sensitive value. If the `for_each` is not
iterable (it is not iterable if marked), a diagnostic containing
the expression context of the `for_each` is created and written
to stderr by the `DiagnosticTextWriter`.

Hopefully this will be fixed in upstream, but here's a workaround
to prevent the panic by excluding expression context from the
diagnostic when the `for_each` is marked.

By the way, other cases that include expression context in
diagnostics do not have the same issue, so no action is required.

- `for_each` is marked in dynamic blocks
  - The value is always unmarked, and non-iterable cases are rarer than those in meta-arguments
- dynamic block labels
  - Dynamic block labels are rarely used in Terraform
- `count` in meta-arguments
  - The value is always unmarked