[ELF] sortSymTabSymbols: change vector to SmallVector

This function may take ~1% time. SmallVector<SymbolTableEntry, 0> is smaller (16 bytes
instead of 24) and more efficient.

lld/ELF/SyntheticSections.cpp

@@ -2127,12 +2127,12 @@ void SymbolTableBaseSection::sortSymTabSymbols() {
   // symbols, they are already naturally placed first in each group. That
   // happens because STT_FILE is always the first symbol in the object and hence
   // precede all other local symbols we add for a file.
-  MapVector<InputFile *, std::vector<SymbolTableEntry>> arr;
+  MapVector<InputFile *, SmallVector<SymbolTableEntry, 0>> arr;
   for (const SymbolTableEntry &s : llvm::make_range(symbols.begin(), e))
     arr[s.sym->file].push_back(s);
 
   auto i = symbols.begin();
-  for (std::pair<InputFile *, std::vector<SymbolTableEntry>> &p : arr)
+  for (auto &p : arr)
     for (SymbolTableEntry &entry : p.second)
       *i++ = entry;
 }