Some HCL callers make the (reasonable) assumption that the overall source
range of an expression will be a superset of all of the ranges of its
child expressions, for purposes such as extraction of source code
snippets, parse tree annotation in hclwrite, text editor analysis
functions like "go to reference", etc.
The IndexExpr type was not previously honoring that assumption, since its
source range was placed around only the bracket portion. That is a good
region to use when reporting errors relating to the index operation, but
it is not a faithful representation of the full extent of the expression.
In order to meet both of these requirements at once, IndexExpr now has
both SrcRange covering the entire expression and BracketRange covering
the index part delimited by brackets. We can then use BracketRange in
our error messages but return SrcRange as the result of the general
Range method that is common to all expression types.
Our error message for the ambiguous situation recommends doing this, but
the parser didn't actually previously allow it. Now we'll accept the form
that the error message recommends.
As before, we also accept a template with an interpolation sequence as
a disambiguation, but the error message doesn't mention that because it's
no longer idiomatic to use an inline string template containing just a
single interpolation sequence.
