compiler: include selected constant types during export processing

compiler: include selected constant types during export processing

    The machinery that collects types referenced by expressions that are
    part of inlinable function bodies was missing the types of local named
    constants in certain cases. This patch updates the
    Collect_export_references::expression() hook to look for references to
    local named constants and include their types in the exported set.

    Fixes golang/go#34577.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/198017

From-SVN: r276594