[lld] Fix use-after-free bug identified by the Address Sanitizer

[garious]

Summary

atomContent's memory is freed at the end of the stack frame, but it is referenced by the atom pushed into _definedAtoms.

[ruiu]

LGTM

lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp

 // the former.
 std::error_code FileCOFF::maybeCreateSXDataAtoms() {
   ArrayRef<uint8_t> sxdata;
   if (std::error_code ec = getSectionContents(".sxdata", sxdata))
     return ec;
   if (sxdata.empty())
     return std::error_code();
 
-  std::vector<uint8_t> atomContent =
-      *new (_alloc) std::vector<uint8_t>((size_t)sxdata.size());
   auto *atom = new (_alloc) COFFDefinedAtom(
       *this, "", ".sxdata", Atom::scopeTranslationUnit, DefinedAtom::typeData,
       false /*isComdat*/, DefinedAtom::permR__, DefinedAtom::mergeNo,
-      atomContent, _ordinal++);
+      sxdata, _ordinal++);
 
   const ulittle32_t *symbolIndex =
       reinterpret_cast<const ulittle32_t *>(sxdata.data());
   int numSymbols = sxdata.size() / sizeof(uint32_t);
 
   for (int i = 0; i < numSymbols; ++i) {
     Atom *handlerFunc;
     if (std::error_code ec = getAtomBySymbolIndex(symbolIndex[i], handlerFunc))