[clang] Add assertions for the smart pointers with the possibility to be null in InMemoryFileSystem::addFile

[OikawaKirie]

Summary

Split from D91844

The parameter variable Buffer in function InMemoryFileSystem::addFile in file llvm/lib/Support/VirtualFileSystem.cpp. The assertion in this function (assert(!(HardLinkTarget && Buffer))) only checks whether these two parameters can both be non-null. But It can be inferred that both pointers can be null together. A null Buffer pointer can be dereferenced without a check.

llvm/lib/Support/VirtualFileSystem.cpp

while (true) {
     ++I;
     if (!Node) {
       if (I == E) {
         // End of the path.
         std::unique_ptr<detail::InMemoryNode> Child;
         if (HardLinkTarget)
           Child.reset(new detail::InMemoryHardLink(P.str(), *HardLinkTarget));
         else {
+          assert(Buffer && "Non-HardLink without a buffer?");
           // Create a new file or directory.
           Status Stat(P.str(), getNextVirtualUniqueID(),
                       llvm::sys::toTimePoint(ModificationTime), ResolvedUser,
                       ResolvedGroup, Buffer->getBufferSize(), ResolvedType,
                       ResolvedPerms);
           if (ResolvedType == sys::fs::file_type::directory_file) {
             Child.reset(new detail::InMemoryDirectory(std::move(Stat)));
           } else {
if (auto *NewDir = dyn_cast<detail::InMemoryDirectory>(Node)) {
       assert((isa<detail::InMemoryFile>(Node) ||
               isa<detail::InMemoryHardLink>(Node)) &&
              "Must be either file, hardlink or directory!");
 
       // Trying to insert a directory in place of a file.
       if (I != E)
         return false;
 
+      assert(Buffer);
       // Return false only if the new file is different from the existing one.
       if (auto Link = dyn_cast<detail::InMemoryHardLink>(Node)) {
         return Link->getResolvedFile().getBuffer()->getBuffer() ==
                Buffer->getBuffer();
       }
       return cast<detail::InMemoryFile>(Node)->getBuffer()->getBuffer() ==
              Buffer->getBuffer();
     }