Refactor GdbIndexSection. NFC.

This patch merges createGdbIndex function and GdbIndexSection's
constructor into a single static member function of the class.

This patch also change how we keep CU vectors. Previously, CuVector
and GdbSymbols were parallel arrays, but there's no reason to choose that
design. Now, CuVector is a member of GdbSymbol class.

A lot of members are removed from GdbIndexSection. Previously, it has
members that need to be kept in sync over several phases. I belive the new
design is less error-prone, and the new code is much easier to read
than before.

llvm-svn: 336743