compiler: improve type handling for string concat ops on constants

compiler: improve type handling for string concat ops on constants

    Resolve a small problem with concatenation of string constants: in a
    string concat X + Y where X has named type and Y has abstract string
    type, insure that the result has X's type, and disable folding if the
    both sides have a concrete type that does not match.

    Fixes golang/go#31412.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/171797

From-SVN: r270336