[clangd] Use function pointer instead of function_ref to avoid GCC 5 bug

With GCC <6 constructing a function_ref from a free function reference
leads to it referencing a temporary function pointer. If the lifetime of
that temporary is insufficient it can crash.

Fixes https://github.com/clangd/clangd/issues/800

clang-tools-extra/clangd/GlobalCompilationDatabase.cpp

@@ -279,11 +279,10 @@ bool DirectoryBasedGlobalCompilationDatabase::DirectoryCache::load(
   struct CDBFile {
     CachedFile *File;
     // Wrapper for {Fixed,JSON}CompilationDatabase::loadFromBuffer.
-    llvm::function_ref<std::unique_ptr<tooling::CompilationDatabase>(
+    std::unique_ptr<tooling::CompilationDatabase> (*Parser)(
         PathRef,
         /*Data*/ llvm::StringRef,
-        /*ErrorMsg*/ std::string &)>
-        Parser;
+        /*ErrorMsg*/ std::string &);
   };
   for (const auto &Entry : {CDBFile{&CompileCommandsJson, parseJSON},
                             CDBFile{&BuildCompileCommandsJson, parseJSON},