[lld][ELF] Add --why-live flag (inspired by Mach-O) (#127112)

This prints a stack of reasons that symbols that match the given glob(s)
survived GC. It has no effect unless section GC occurs.

This implementation does not require -ffunction-sections or
-fdata-sections to produce readable results, althought it does tend to
work better (as does GC).

Details about the semantics:
- Some chain of liveness reasons is reported; it isn't specified which
chain.
 - A symbol or section may be live:
   - Intrisically (e.g., entry point)
   - Because needed by a live symbol or section
   - (Symbols only) Because part of a section live for another reason
   - (Sections only) Because they contain a live symbol
 - Both global and local symbols (`STB_LOCAL`) are supported.
 - References to symbol + offset are considered to point to:
   - If the referenced symbol is a section (`STT_SECTION`):
- If a sized symbol encloses the referenced offset, the enclosing
symbol.
     - Otherwise, the section itself, generically.
   - Otherwise, the referenced symbol.

Author: mysterymath

lld/ELF/Config.h

@@ -226,6 +226,7 @@ struct Config {
   llvm::StringRef thinLTOCacheDir;
   llvm::StringRef thinLTOIndexOnlyArg;
   llvm::StringRef whyExtract;
+  llvm::SmallVector<llvm::GlobPattern, 0> whyLive;
   llvm::StringRef cmseInputLib;
   llvm::StringRef cmseOutputLib;
   ReportPolicy zBtiReport = ReportPolicy::None;

lld/ELF/Driver.cpp

@@ -1545,6 +1545,15 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   ctx.arg.warnSymbolOrdering =
       args.hasFlag(OPT_warn_symbol_ordering, OPT_no_warn_symbol_ordering, true);
   ctx.arg.whyExtract = args.getLastArgValue(OPT_why_extract);
+  for (opt::Arg *arg : args.filtered(OPT_why_live)) {
+    StringRef value(arg->getValue());
+    if (Expected<GlobPattern> pat = GlobPattern::create(arg->getValue())) {
+      ctx.arg.whyLive.emplace_back(std::move(*pat));
+    } else {
+      ErrAlways(ctx) << arg->getSpelling() << ": " << pat.takeError();
+      continue;
+    }
+  }
   ctx.arg.zCombreloc = getZFlag(args, "combreloc", "nocombreloc", true);
   ctx.arg.zCopyreloc = getZFlag(args, "copyreloc", "nocopyreloc", true);
   ctx.arg.zForceBti = hasZOption(args, "force-bti");