[clangd] Enable clang-tidy by default.

[hokein]

Summary

We have turned on the flag internally for a while, and we don't receive complains.
Should be good to turn it on now.

If the projects doesn't have .clang-tidy files, no clang-tidy check will
be run.

[ilya-biryukov]

Yay! LGTM

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

static llvm::cl::opt<std::string> ClangTidyChecks(
     llvm::cl::desc(
         "List of clang-tidy checks to run (this will override "
         ".clang-tidy files). Only meaningful when -clang-tidy flag is on."),
     llvm::cl::init(""));
 
 static llvm::cl::opt<bool> EnableClangTidy(
     "clang-tidy",
     llvm::cl::desc("Enable clang-tidy diagnostics."),
-    llvm::cl::init(false));
+    llvm::cl::init(true));
 
 static llvm::cl::opt<bool> SuggestMissingIncludes(
     "suggest-missing-includes",
     llvm::cl::desc("Attempts to fix diagnostic errors caused by missing "
                    "includes using index."),
     llvm::cl::init(true));
 
 static llvm::cl::opt<OffsetEncoding> ForceOffsetEncoding(