Use correct sign in get_mpz

This changes dwarf2/read.c:get_mpz to use the correct sign-extension
function.  Normally a rational constant uses signed values, but a
purely unsigned form also seems fine here.  This adds a new
attribute::form_is_strictly_unsigned, which is more precise than
form_is_unsigned (which accepts a lot of forms that aren't really for
ordinary constants).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680

Author: tromey

gdb/dwarf2/attribute.h

@@ -175,6 +175,15 @@ struct attribute
      false.  */
   bool form_is_strictly_signed () const;
 
+  /* Check if the attribute's form is an unsigned constant form.  This
+     only returns true for forms that are strictly unsigned -- that
+     is, for a context-dependent form like DW_FORM_data1, this returns
+     false.  */
+  bool form_is_strictly_unsigned () const
+  {
+    return form == DW_FORM_udata;
+  }
+
   /* Check if the attribute's form is a form that requires
      "reprocessing".  */
   bool form_requires_reprocessing () const;

gdb/dwarf2/read.c

@@ -13216,10 +13216,10 @@ get_mpz (struct dwarf2_cu *cu, gdb_mpz *value, struct attribute *attr)
 		   ? BFD_ENDIAN_BIG : BFD_ENDIAN_LITTLE,
 		   true);
     }
-  else if (attr->form_is_unsigned ())
+  else if (attr->form_is_strictly_unsigned ())
     *value = gdb_mpz (attr->as_unsigned ());
   else
-    *value = gdb_mpz (attr->constant_value (1));
+    *value = gdb_mpz (attr->signed_constant ().value_or (1));
 }
 
 /* Assuming DIE is a rational DW_TAG_constant, read the DIE's