[clangd] Mention whether compile flags were inferred in check mode

[kadircet]

Summary

That way when looking at logs it's clear whether diagnostics are a
result of compile flags mismatch.

[ilya-biryukov]

LGTM.
Please note the NIT that arguably improves readability.

clang-tools-extra/clangd/tool/Check.cpp

114–116

NIT: maybe change the order of arguments and the format string placeholders?
it's a bit hard to swap arguments inside one's head when reading the code.

log("{1}, {0}!", "world", "Hello");
// vs
log("{0}, {1}!", "Hello", "world");

clang-tools-extra/clangd/tool/Check.cpp

bool buildCommand(const ThreadsafeFS &TFS) {
     if (Opts.ResourceDir)
       Mangler.ResourceDir = *Opts.ResourceDir;
     auto CDB = std::make_unique<OverlayCDB>(
         BaseCDB.get(), std::vector<std::string>{},
         tooling::ArgumentsAdjuster(std::move(Mangler)));
 
     if (auto TrueCmd = CDB->getCompileCommand(File)) {
       Cmd = std::move(*TrueCmd);
-      log("Compile command from CDB is: {0}", printArgv(Cmd.CommandLine));
+      log("Compile command {0} is: {1}",
+          Cmd.Heuristic.empty() ? "from CDB" : Cmd.Heuristic,
+          printArgv(Cmd.CommandLine));
     } else {
       Cmd = CDB->getFallbackCommand(File);
       log("Generic fallback command is: {0}", printArgv(Cmd.CommandLine));
     }
 
     return true;
   }