[DebugInfo]: Allow DwarfCompileUnit to have line table symbol

[SouraVX]

Summary

Previously line table symbol was represented as DIE::value_iterator inside DwarfCompileUnit and subsequent function intStmtList was used to create a local MCSymbol to initialize it.

This patch removes DIE::value_iterator from DwarfCompileUnit and intoduce MCSymbol for representing this units symbol for debug_line section. As a result applyStmtList is also modified to utilize this. Further more a helper function getLineTableStartSym is also introduced to get this symbol, this would be used by clients which need to access this line table, i.e debug_macro.

This patch is based on @dblaikie comment https://reviews.llvm.org/D72828#1958625 on D72828
Testing - No regression found on trunk.

[dblaikie]

Looks good - thanks!

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h

 
 enum class UnitKind { Skeleton, Full };
 
 class DwarfCompileUnit final : public DwarfUnit {
   /// A numeric ID unique among all CUs in the module
   unsigned UniqueID;
   bool HasRangeLists = false;
 
-  /// The attribute index of DW_AT_stmt_list in the compile unit DIE, avoiding
-  /// the need to search for it in applyStmtList.
-  DIE::value_iterator StmtListValue;
+  /// The start of the unit line section, this is also
+  /// reused in appyStmtList.
+  MCSymbol *LineTableStartSym;
 
   /// Skeleton unit associated with this unit.
   DwarfCompileUnit *Skeleton = nullptr;
 
   /// The start of the unit within its section.
   MCSymbol *LabelBegin;
 
   /// The start of the unit macro info within macro section.
public:
 
   bool includeMinimalInlineScopes() const;
 
   void initStmtList();
 
   /// Apply the DW_AT_stmt_list from this compile unit to the specified DIE.
   void applyStmtList(DIE &D);
 
+  /// Get line table start symbol for this unit.
+  MCSymbol *getLineTableStartSym() const { return LineTableStartSym; }
+
   /// A pair of GlobalVariable and DIExpression.
   struct GlobalExpr {
     const GlobalVariable *Var;
     const DIExpression *Expr;
   };
 
   struct BaseTypeRef {
     BaseTypeRef(unsigned BitSize, dwarf::TypeKind Encoding) :

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

void DwarfCompileUnit::addRange(RangeSpan Range) {
 
   CURanges.back().End = Range.End;
 }
 
 void DwarfCompileUnit::initStmtList() {
   if (CUNode->isDebugDirectivesOnly())
     return;
 
-  // Define start line table label for each Compile Unit.
-  MCSymbol *LineTableStartSym;
   const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();
   if (DD->useSectionsAsReferences()) {
     LineTableStartSym = TLOF.getDwarfLineSection()->getBeginSymbol();
   } else {
     LineTableStartSym =
         Asm->OutStreamer->getDwarfLineTableSymbol(getUniqueID());
   }
 
   // DW_AT_stmt_list is a offset of line number information for this
   // compile unit in debug_line section. For split dwarf this is
   // left in the skeleton CU and so not included.
   // The line table entries are not always emitted in assembly, so it
   // is not okay to use line_table_start here.
-  StmtListValue =
       addSectionLabel(getUnitDie(), dwarf::DW_AT_stmt_list, LineTableStartSym,
                       TLOF.getDwarfLineSection()->getBeginSymbol());
 }
 
 void DwarfCompileUnit::applyStmtList(DIE &D) {
-  D.addValue(DIEValueAllocator, *StmtListValue);
+  const TargetLoweringObjectFile &TLOF = Asm->getObjFileLowering();
+  addSectionLabel(D, dwarf::DW_AT_stmt_list, LineTableStartSym,
+                  TLOF.getDwarfLineSection()->getBeginSymbol());
 }
 
 void DwarfCompileUnit::attachLowHighPC(DIE &D, const MCSymbol *Begin,
                                        const MCSymbol *End) {
   assert(Begin && "Begin label should not be null!");
   assert(End && "End label should not be null!");
   assert(Begin->isDefined() && "Invalid starting label");
   assert(End->isDefined() && "Invalid end label");