[clangd][tests] Fix handling of EOF in delimited input

[jkorous-apple]

Summary

Request in delimited input ended by EOF shouldn't be an error state.
Comments should be allowed at the end of test files.
Input mirroring should work for the last request in delimited test file.

JSONRPCDispatcher.cpp

static llvm::Optional<std::string> readDelimitedMessage(std::istream &In,
   std::string Line;
   while (std::getline(In, Line)) {
     Line.push_back('\n'); // getline() consumed the newline.
 
     auto LineRef = llvm::StringRef(Line).trim();
     if (LineRef.startswith("#")) // comment
       continue;
 
-    bool IsDelim = LineRef.find_first_not_of('-') == llvm::StringRef::npos;
-    if (!IsDelim) // Line is part of a JSON message.
-      JSON += Line;
-    if (IsDelim) {
-      Out.mirrorInput(
-          llvm::formatv("Content-Length: {0}\r\n\r\n{1}", JSON.size(), JSON));
-      return std::move(JSON);
-    }
+    // found a delimiter
+    if (LineRef.find_first_not_of('-') == llvm::StringRef::npos)
+      break;
+
   }
 
   if (In.bad()) {
     log("Input error while reading message!");
     return llvm::None;
   } else {
-    log("Input message terminated by EOF");
+    Out.mirrorInput(
+        llvm::formatv("Content-Length: {0}\r\n\r\n{1}", JSON.size(), JSON));
     return std::move(JSON);
   }
 }
 
 void clangd::runLanguageServerLoop(std::istream &In, JSONOutput &Out,
                                    JSONStreamStyle InputStyle,
                                    JSONRPCDispatcher &Dispatcher,
                                    bool &IsDone) {

clangd/delimited-input-comment-at-the-end.test

+# RUN: clangd -input-style=delimited -run-synchronously -input-mirror-file %t < %s
+# RUN: grep '{"jsonrpc":"2.0","id":3,"method":"exit"}' %t
+#
+# RUN: clangd -lit-test -input-mirror-file %t < %s
+# RUN: grep '{"jsonrpc":"2.0","id":3,"method":"exit"}' %t
+#
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+---
+{"jsonrpc":"2.0","id":3,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","id":3,"method":"exit"}
+# comment at the end