[clangd] Fail more gracefully if QueryDriverDatabase cannot determine file type

[nridge]

Summary

Currently, QueryDriverDatabase returns an empty compile command
if it could not determine the file type.

This failure mode is unnecessarily destructive; it's better to
just return the incoming compiler command, which is still more
likely to be useful than an empty command.

clang-tools-extra/clangd/QueryDriverDatabase.cpp

for (size_t I = 0, E = Cmd->CommandLine.size(); I < E; ++I) {
       else if (Arg.startswith("-x"))
         Lang = Arg.drop_front(2).trim();
     }
     if (Lang.empty()) {
       llvm::StringRef Ext = llvm::sys::path::extension(File).trim('.');
       auto Type = driver::types::lookupTypeForExtension(Ext);
       if (Type == driver::types::TY_INVALID) {
         elog("System include extraction: invalid file type for {0}", Ext);
-        return {};
+        return Cmd;
       }
       Lang = driver::types::getTypeName(Type);
     }
 
     llvm::SmallString<128> Driver(Cmd->CommandLine.front());
     if (llvm::any_of(Driver,
                        [](char C) { return llvm::sys::path::is_separator(C); }))
       // Driver is a not a single executable name but instead a path (either