compiler: Avoid knock-on errors with impossibly large types.

compiler: Avoid knock-on errors with impossibly large types.

    The gofrontend asks the backend compiler for the architecture
    appropriate size of a given type.  For array types, it is possible
    to construct a type too large to fit on the machine.  This patch does
    two things: 1. When an impossibly large type is encountered, we mark
    the type as erroneous and later calls to discover the size of that
    type are short-circuited. 2. When generating the GC symbol data for
    an impossibly large array of arrays, we avoid generating symbol data
    as soon as we find an array that is too large to be expressed.

    Fixes golang/go#12938.

    Reviewed-on: https://go-review.googlesource.com/16234

From-SVN: r232855