Use attribute::signed_constant in attribute::as_boolean

This changes attribute::as_boolean to use attribute::signed_constant.
This is maybe overkill but lets any reasonable constant form through.

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

Author: tromey

gdb/dwarf2/attribute.c

@@ -326,5 +326,8 @@ attribute::as_boolean () const
     return true;
   else if (form == DW_FORM_flag)
     return u.unsnd != 0;
-  return constant_value (0) != 0;
+  /* Using signed_constant here will work even for the weird case
+     where a negative value is provided.  Probably doesn't matter but
+     also seems harmless.  */
+  return signed_constant ().value_or (0) != 0;
 }