[clangd][NFC] Small tweak to combined provider

[njames93]

Summary

This should address the FIXME about clang3.9 dervied to base unique_ptr constructor not working.

[njames93]

Removed const qualifier on Providers.

[sammccall]

(I'm not sure how it resolves the problem, but certainly looks nicer!)

[njames93]

In D91925#2439666, @sammccall wrote:

(I'm not sure how it resolves the problem, but certainly looks nicer!)

It's something to do with the derived to base unique_ptr conversion not working properly with NRVO.
However removing the named variable should elide that, and as an added bonus it does look neater.

clang-tools-extra/clangd/ConfigProvider.cpp

public:
     }
   };
 
   return std::make_unique<RelFileProvider>(RelPath, FS);
 }
 
 std::unique_ptr<Provider>
 Provider::combine(std::vector<const Provider *> Providers) {
-  struct CombinedProvider : Provider {
-    std::vector<const Provider *> Providers;
+  class CombinedProvider : public Provider {
+    const std::vector<const Provider *> Providers;
 
     std::vector<CompiledFragment>
     getFragments(const Params &P, DiagnosticCallback DC) const override {
       std::vector<CompiledFragment> Result;
       for (const auto &Provider : Providers) {
         for (auto &Fragment : Provider->getFragments(P, DC))
           Result.push_back(std::move(Fragment));
       }
       return Result;
     }
+
+  public:
+    CombinedProvider(std::vector<const Provider *> Providers)
+        : Providers(std::move(Providers)) {}
   };
-  auto Result = std::make_unique<CombinedProvider>();
-  Result->Providers = std::move(Providers);
-  // FIXME: This is a workaround for a bug in older versions of clang (< 3.9)
-  //   The constructor that is supposed to allow for Derived to Base
-  //   conversion does not work. Remove this if we drop support for such
-  //   configurations.
-  return std::unique_ptr<Provider>(Result.release());
+
+  return std::make_unique<CombinedProvider>(std::move(Providers));
 }
 
 Config Provider::getConfig(const Params &P, DiagnosticCallback DC) const {
   trace::Span Tracer("getConfig");
   if (!P.Path.empty())
     SPAN_ATTACH(Tracer, "path", P.Path);
   Config C;
   for (const auto &Fragment : getFragments(P, DC))
     Fragment(P, C);
   return C;
 }
 
 } // namespace config
 } // namespace clangd
 } // namespace clang