[ELF] --gdb-index: error if constant pool size exceeds UINT32_MAX

If so, the last symbol's name_offset likely exceeds 0xffffffff and is not
supported by the format
(https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html#Index-Section-Format).
I have seen an internal oversized executable with such a corrupted .gdb_index

lld/ELF/SyntheticSections.cpp

@@ -2844,6 +2844,10 @@ createSymbols(
     sym.nameOff = off;
     off += sym.name.size() + 1;
   }
+  // If off overflows, the last symbol's nameOff likely overflows.
+  if (!isUInt<32>(off))
+    errorOrWarn("--gdb-index: constant pool size (" + Twine(off) +
+                ") exceeds UINT32_MAX");
 
   return {ret, off};
 }