Fixes https://github.com/hashicorp/terraform/issues/6359 and several
related issues by unescaping a double backslash within braces to a
single backslash.
This bumps into the bit of HIL that still hangs out in HCL - for values
that aren't interpolated, like Terraform's variable defaults - users
were unable to get a backslash to show up within `${}`.
That's because `${}` is handled specially to allow for, e.g., double
quotes inside of braces.
Here, we add `\\` as a special cased escape (along with `\"`) so users
can get backslashes in these scenarios by doubling them up.
8e05f061d6