compiler: Mark erroneous constants as invalid.
When the compiler failed to evaluate a numeric constant because
because it was erroneous, there was no way for parent nodes to
discover the error and lower themselves into error nodes. This patch
now uses the NC_INVALID enumerator to mark numeric constants with a
known, reported error.
Fixes golang/go#11541.
Reviewed-on: https://go-review.googlesource.com/13904
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@227420 138bc75d-0d04-0410-961f-82ee72b054a4