[ELF] - Add testcase testing .gdb_index generation when base address of CU is used.

Recently (before r312477) lib/DebugInfo incorrectly handled the case
when debug ranges uses base address of CU. Section index was not
calulcated properly and LLD crashed on provided testcase.

Differential revision: https://reviews.llvm.org/D37297

llvm-svn: 312478