[BOLT] Refactor undefined symbols handling. NFCI (#167075)

Remove internal undefined symbol tracking and instead rely on the
emission state of `MCSymbol` while processing data-to-code relocations.

Note that `CleanMCState` pass resets the state of all `MCSymbol`s prior
to code emission.

Author: maksfb

bolt/include/bolt/Core/BinaryContext.h

@@ -354,9 +354,6 @@ class BinaryContext {
   /// Newly created segments.
   std::vector<SegmentInfo> NewSegments;
 
-  /// Symbols that are expected to be undefined in MCContext during emission.
-  std::unordered_set<MCSymbol *> UndefinedSymbols;
-
   /// [name] -> [BinaryData*] map used for global symbol resolution.
   using SymbolMapType = StringMap<BinaryData *>;
   SymbolMapType GlobalSymbols;

bolt/lib/Core/BinaryFunction.cpp

@@ -1896,16 +1896,6 @@ bool BinaryFunction::scanExternalRefs() {
     }
   }
 
-  // Inform BinaryContext that this function symbols will not be defined and
-  // relocations should not be created against them.
-  if (BC.HasRelocations) {
-    for (std::pair<const uint32_t, MCSymbol *> &LI : Labels)
-      BC.UndefinedSymbols.insert(LI.second);
-    for (MCSymbol *const EndLabel : FunctionEndLabels)
-      if (EndLabel)
-        BC.UndefinedSymbols.insert(EndLabel);
-  }
-
   clearList(Relocations);
   clearList(ExternallyReferencedOffsets);
 
@@ -3234,14 +3224,6 @@ void BinaryFunction::clearDisasmState() {
   clearList(Instructions);
   clearList(IgnoredBranches);
   clearList(TakenBranches);
-
-  if (BC.HasRelocations) {
-    for (std::pair<const uint32_t, MCSymbol *> &LI : Labels)
-      BC.UndefinedSymbols.insert(LI.second);
-    for (MCSymbol *const EndLabel : FunctionEndLabels)
-      if (EndLabel)
-        BC.UndefinedSymbols.insert(EndLabel);
-  }
 }
 
 void BinaryFunction::setTrapOnEntry() {

bolt/lib/Core/BinarySection.cpp

@@ -112,8 +112,10 @@ void BinarySection::emitAsData(MCStreamer &Streamer,
       RI = ROE;
 
       // Skip undefined symbols.
-      auto HasUndefSym = [this](const auto &Relocation) {
-        return BC.UndefinedSymbols.count(Relocation.Symbol);
+      auto HasUndefSym = [](const auto &Relocation) {
+        return Relocation.Symbol && Relocation.Symbol->isTemporary() &&
+               Relocation.Symbol->isUndefined() &&
+               !Relocation.Symbol->isRegistered();
       };
 
       if (std::any_of(ROI, ROE, HasUndefSym))