compiler: Flatten erroneous subtrees into errors.

compiler: Flatten erroneous subtrees into errors.

    Between the lowering and flattening passes of the compiler, there are
    several passes that modify the lowered Go parse tree and as errors are
    discovered, several nodes transform into error nodes.  However, for a
    higher level node such as a construction expression, the erroneous
    nodes in the subtrees might not propagate their error.  The flatten
    phase for a node now looks for errors in the subtree and flattens the
    node into an error node if any are found.

    Fixes golang/go#11559, golang/go#11536, golang/go#11558.

    Reviewed-on: https://go-review.googlesource.com/13097

From-SVN: r226845