Resolve FIXMEs: cosmetic / speculative / near-dead markers

Low impact. Several are "verify then maybe delete"; confirm no refs first.

• include/eld/Support/INIReader.h:29 — "make this private"
• include/eld/Script/OutputSectData.h:55 — "Ideally, it should be private."
• include/eld/Input/BitcodeFile.h:70 — "Used by BitcodeReader, may not be needed."
• lib/Core/Module.cpp:340 — "Test if it's needed at all."
• lib/Diagnostics/DiagnosticEngine.cpp:214 — "currently there are fewer diagnostic..."
• include/eld/Input/InputTree.h:86 — "Convert to std::optional"
• lib/Config/LinkerConfig.cpp:64 — "Use DIAG here."
• lib/Support/INIReader.cpp:25 — "Check for errors."
• lib/LinkerWrapper/DWARF.cpp:25 — bare "Remove"
• lib/Target/X86/x86_64Info.cpp:56 — bare // FIXME
• lib/Target/AArch64/AArch64LDBackend.h:180 — bare /// FIXME:
• include/eld/PluginAPI/PluginADT.h:783 — bare // TODO:
• include/eld/PluginAPI/PluginADT.h:1252 — bare /// TODO
• lib/Target/Template/TemplateInfo.cpp:24 — "Add proper code."
• lib/Plugin/PluginActivityLog.cpp:110 — "This needs to be completed."