[clang-tools-extra] [clangd] Make clangd-indexer respect .clangd configuration files (PR #214951)

[Nathan Ridge via cfe-commits]

================
@@ -152,6 +166,17 @@ int main(int argc, const char **argv) {
     return 1;
   }
 
+  clang::clangd::RealThreadsafeFS TFS;
+  std::vector<std::unique_ptr<clang::clangd::config::Provider>> ProviderStack;
+  if (clang::clangd::EnableConfig)
+    ProviderStack =
+        clang::clangd::config::Provider::createDefaultProviders(TFS);
+  auto ConfigProvider =
+      clang::clangd::config::Provider::combineOwned(std::move(ProviderStack));
+  auto ContextProvider =
+      clang::clangd::ClangdServer::createConfiguredContextProvider(
----------------
HighCommander4 wrote:

Passing `Callbacks=nullptr` here means that diagnostics encountered while parsing the config file will be silently dropped.

As mentioned [in the Discourse thread](https://discourse.llvm.org/t/honor-clangd-compileflags-overrides-in-clangd-indexer/91504/3?u=highcommander4), I think the behaviour we'd want here is to print the diagnostics to the terminal.

There are two ways we could accomplish this:

 * Give the indexer its own `ClangdServer::Callbacks` implementation and pass that to `ClangdServer::createConfiguredcontextProvider` here
 * Give the indexer its own (simpler) "create context provider from config provider" function and use that instead of `ClangdServer::createConfiguredContextProvider`  

Not sure which one is less hassle (maybe the latter).

https://github.com/llvm/llvm-project/pull/214951