[clang-doc] Fix warnings about lock_guard.

[brettw]

Summary

Fixes a warning about a potentially unsupported template argument deduction by explicitly specifying the template type in std::lock_guard.

[phosek]

LGTM

clang-tools-extra/clang-doc/tool/ClangDocMain.cpp

Pool.async([&]() {
       auto Reduced = doc::mergeInfos(Infos);
       if (!Reduced) {
         llvm::errs() << llvm::toString(Reduced.takeError());
         return;
       }
 
       // Add a reference to this Info in the Index
       {
-        std::lock_guard Guard(IndexMutex);
+        std::lock_guard<llvm::sys::Mutex> Guard(IndexMutex);
         clang::doc::Generator::addInfoToIndex(CDCtx.Idx, Reduced.get().get());
       }
 
       // Save in the result map (needs a lock due to threaded access).
       {
-        std::lock_guard Guard(USRToInfoMutex);
+        std::lock_guard<llvm::sys::Mutex> Guard(USRToInfoMutex);
         USRToInfo[Group.getKey()] = std::move(Reduced.get());
       }
     });
   }
 
   Pool.wait();
 
   if (Error)