[ELF] - Do not produce duplicate values in .gdb_index's constant pool area.

I found this when builded llc binary using gcc 5.4.1 + LLD.
gcc produces duplicate entries in .debug_gnu_pubtypes section, ex:

UnifyFunctionExitNodes.cpp.o has:
0x0000ac07 EXTERNAL TYPE "std::success_type<void*>"
0x0000ac07 EXTERNAL TYPE "std::success_type<void*>"

clang produces single entry here:
0x0000d291 EXTERNAL TYPE "std::__success_type<void *>"

If we link output from gcc with LLD, that would produce excessive duplicate
entries in .gdb_index constant pool area. That does not seem affect gdb work,
but makes .gdb_index larger than it can be.

I also checked that gold filters out such duplicates too. Patch fixes it.

Differential revision: https://reviews.llvm.org/D32647

llvm-svn: 303975