[clangd] Move logging out of LSPTest base class into a separate one.

[kadircet]

Summary

This was causing TSan failures due to a race on vptr during destruction,
see
https://lab.llvm.org/buildbot/#/builders/131/builds/6579/steps/6/logs/FAIL__Clangd_Unit_Tests__LSPTest_FeatureModulesThr.

The story is, during the execution of a destructor all the virtual
dispatches should resolve to implementations in the class being
destroyed, not the derived ones. And LSPTests will log some stuff during
destruction (we send shutdown/exit requests, which are logged), which is
a virtual dispatch when LSPTest is derived from clang::clangd::Logger.

It is a benign race in our case, as gtests that derive from LSPTest
doesn't override the log method. But still during destruction, we
might try to update vtable ptr (due to being done with destruction of
test and starting destruction of LSPTest) and read from it to dispatch a
log message at the same time.

This patch fixes that race by moving log out of the vtable of
LSPTest.

[sammccall]

Thanks for working this out!

And sorry for the stupid private inheritance tricks :-(

clang-tools-extra/clangd/unittests/ClangdLSPServerTests.cpp

 MATCHER_P(DiagMessage, M, "") {
   if (const auto *O = arg.getAsObject()) {
     if (const auto Msg = O->getString("message"))
       return *Msg == M;
   }
   return false;
 }
 
-class LSPTest : public ::testing::Test, private clangd::Logger {
+class LSPTest : public ::testing::Test {
 protected:
-  LSPTest() : LogSession(*this) {
+  LSPTest() : LogSession(L) {
     ClangdServer::Options &Base = Opts;
     Base = ClangdServer::optsForTest();
     // This is needed to we can test index-based operations like call hierarchy.
     Base.BuildDynamicSymbolIndex = true;
     Base.FeatureModules = &FeatureModules;
   }
 
   LSPClient &start() {
if (Server)
       stop();
   }
 
   MockFS FS;
   ClangdLSPServer::Options Opts;
   FeatureModuleSet FeatureModules;
 
 private:
+  class Logger : public clang::clangd::Logger {
     // Color logs so we can distinguish them from test output.
     void log(Level L, const char *Fmt,
              const llvm::formatv_object_base &Message) override {
       raw_ostream::Colors Color;
       switch (L) {
       case Level::Verbose:
         Color = raw_ostream::BLUE;
         break;
       case Level::Error:
         Color = raw_ostream::RED;
         break;
       default:
         Color = raw_ostream::YELLOW;
         break;
       }
       std::lock_guard<std::mutex> Lock(LogMu);
       (llvm::outs().changeColor(Color) << Message << "\n").resetColor();
     }
     std::mutex LogMu;
+  };
 
+  Logger L;
   LoggingSession LogSession;
   llvm::Optional<ClangdLSPServer> Server;
   llvm::Optional<std::thread> ServerThread;
   LSPClient Client;
 };
 
 TEST_F(LSPTest, GoToDefinition) {
   Annotations Code(R"cpp(