debug info: Refine the flow of handling the .debug_str merging

Move DebguString from Module to output .debug_str LDSection.
The flow of handling debug string merging:
1. ObjectLinker::mergeSections - parse the input .debug_str sections and add the
   strings to DebugString merged table
2. ObjectLinker::preLayout - set the output offset of the debug strings and size
   the output section
3. ObjectLinker::relocation - apply the debug string offset accordings to
   corresponding relocations