[JITLink] Use MapVector to stabilize iteration order

Otherwise LinkGraph::dump output could change
(llvm/test/ExecutionEngine/JITLink/x86-64/COFF_pdata_strip.s) when
llvm::hash_value(StringRef) changes.

llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h

@@ -15,6 +15,7 @@
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/FunctionExtras.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ExecutionEngine/JITLink/JITLinkMemoryManager.h"
@@ -847,7 +848,7 @@ class SectionRange {
 
 class LinkGraph {
 private:
-  using SectionMap = DenseMap<StringRef, std::unique_ptr<Section>>;
+  using SectionMap = MapVector<StringRef, std::unique_ptr<Section>>;
   using ExternalSymbolMap = StringMap<Symbol *>;
   using AbsoluteSymbolSet = DenseSet<Symbol *>;
   using BlockSet = DenseSet<Block *>;
@@ -1543,7 +1544,7 @@ class LinkGraph {
   unsigned PointerSize;
   llvm::endianness Endianness;
   GetEdgeKindNameFunction GetEdgeKindName = nullptr;
-  DenseMap<StringRef, std::unique_ptr<Section>> Sections;
+  MapVector<StringRef, std::unique_ptr<Section>> Sections;
   ExternalSymbolMap ExternalSymbols;
   AbsoluteSymbolSet AbsoluteSymbols;
   orc::shared::AllocActions AAs;