Fix debuginfod build failure when httplib is used. (llvm#156107)

This is a follow up of adbd432. The
problem is, HTTPServer class will lost its implicit move ctor if httplib
is used. This patch adds the move ctor explicitly to solve this issue.
The default move ctor is not added in the header due to a limitation
that "httplib::Server" is a forward declaration and it is incomplete.

Author: zeroomega

llvm/include/llvm/Debuginfod/HTTPServer.h

@@ -104,6 +104,7 @@ class HTTPServer {
 public:
   HTTPServer();
   ~HTTPServer();
+  HTTPServer(HTTPServer &&);
 
   /// Returns true only if LLVM has been compiled with a working HTTPServer.
   static bool isAvailable();

llvm/lib/Debuginfod/HTTPServer.cpp

@@ -62,6 +62,8 @@ bool llvm::streamFile(HTTPServerRequest &Request, StringRef FilePath) {
   return true;
 }
 
+HTTPServer::HTTPServer(HTTPServer &&) = default;
+
 #ifdef LLVM_ENABLE_HTTPLIB
 
 bool HTTPServer::isAvailable() { return true; }