Refactor GdbIndexSection. NFC.

This patch rewrites a part of GdbIndexSection to address the following
issues in the previous implementation:

 - Previously, some struct declarations were in GdbIndex.h while they
   were not used in GdbIndex.cpp. Such structs are moved to
   SyntheticSection.h.

 - The actual implementation were split into GdbIndexSection and GdbHash
   section, but that separation didn't make much sense. They are now
   unified as GdbIndexSection.

In addition to the above changes, this patch splits functions, rename
variables and remove redundant functions/variables to generally improve
code quality.

llvm-svn: 314084