[ELF] - Simplify readAddressArea() implementation.

Approach significantly simplifies LLD .gdb_index code and makes it much faster.
Also it should resolve issues, like D33176 tries to address once and forever in a clean way.

LLC binary linking without patch and without --gdb-index: 1,599241063
LLC binary linking without patch and with --gdb-index: 6,064316262
LLC binary linking with patch and with --gdb-index: 4,116792104

Time spent for building gdbindex changes from (6,064316262 - 1,599241063 == 4,465075199)
to (4,116792104- 1,599241063 == 2,517551041).
That is 2,517551041/4,465075199 = 0,564 or about 44% speedup.

Differential revision: https://reviews.llvm.org/D33183

llvm-svn: 304895