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

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c375f3bf470f94220149b486c947bb3eb57cde7d
+731941c155214d6158fa800e52ab3225c0b55f73
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -2550,6 +2550,8 @@ Type::backend_type_size(Gogo* gogo, int64_t *psize)
 {
   if (!this->is_backend_type_size_known(gogo))
     return false;
+  if (this->is_error_type())
+    return false;
   Btype* bt = this->get_backend_placeholder(gogo);
   *psize = gogo->backend()->type_size(bt);
   if (*psize == -1)
@@ -6453,7 +6455,7 @@ Array_type::slice_gc_symbol(Gogo* gogo, Expression_list** vals,
   (*vals)->push_back(Expression::make_integer_ul(opval, uintptr_type, bloc));
   (*vals)->push_back(*offset);
 
-  if (element_size != 0)
+  if (element_size != 0 && ok)
     (*vals)->push_back(Expression::make_gc_symbol(element_type));
   this->advance_gc_offset(offset);
 }
@@ -6488,7 +6490,7 @@ Array_type::array_gc_symbol(Gogo* gogo, Expression_list** vals,
   Type* element_type = this->element_type();
   if (bound < 1 || !element_type->has_pointer())
     this->advance_gc_offset(offset);
-  else if (bound == 1 || iwidth <= 4 * pwidth)
+  else if (ok && (bound == 1 || iwidth <= 4 * pwidth))
     {
       for (unsigned int i = 0; i < bound; ++i)
 	Type::gc_symbol(gogo, element_type, vals, offset, stack_size);