LibJSGCVerifier: Support message passing between Clang processes

This allows each Clang process to send JSON messages to the
orchestrating Python process, which aggregates the message and can do
something with them all at the end. This is required because we run
Clang multithreaded to speed up the tool execution.

I did try to add a second frontend tool that accepts all the files at
once, but it was _extremely_ slow, so this is the next best thing.

Meta/Lagom/Tools/LibJSGCVerifier/src/CellsHandler.cpp

@@ -124,20 +124,6 @@ CollectCellsHandler::CollectCellsHandler()
         this);
 }
 
-bool CollectCellsHandler::handleBeginSource(clang::CompilerInstance& ci)
-{
-    auto const& source_manager = ci.getSourceManager();
-    auto file_id = source_manager.getMainFileID();
-    auto const* file_entry = source_manager.getFileEntryForID(file_id);
-    if (!file_entry)
-        return false;
-
-    auto current_filepath = std::filesystem::canonical(file_entry->getName().str());
-    llvm::outs() << "Processing " << current_filepath.string() << "\n";
-
-    return true;
-}
-
 bool record_inherits_from_cell(clang::CXXRecordDecl const& record)
 {
     if (!record.isCompleteDefinition())

Meta/Lagom/Tools/LibJSGCVerifier/src/CellsHandler.h

@@ -18,8 +18,6 @@ public:
     CollectCellsHandler();
     virtual ~CollectCellsHandler() override = default;
 
-    virtual bool handleBeginSource(clang::CompilerInstance&) override;
-
     virtual void run(clang::ast_matchers::MatchFinder::MatchResult const& result) override;
 
     clang::ast_matchers::MatchFinder& finder() { return m_finder; }

Meta/Lagom/Tools/LibJSGCVerifier/src/main.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import argparse
+import json
 import multiprocessing
 import os
 from pathlib import Path
@@ -69,14 +70,25 @@ def thread_execute(file_path):
         compile_commands_path,
         file_path
     ]
-    proc = subprocess.Popen(clang_args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-    sys.stdout.buffer.write(proc.communicate()[0])
-    sys.stdout.buffer.flush()
+    proc = subprocess.Popen(clang_args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    stdout, stderr = proc.communicate()
+    print(f'Processed {file_path.resolve()}')
+    if stderr:
+        print(stderr.decode(), file=sys.stderr)
+
+    results = []
+    if stdout:
+        for line in stdout.split(b'\n'):
+            if line:
+                results.append(json.loads(line))
+    return results
 
 
 with multiprocessing.Pool(processes=multiprocessing.cpu_count() - 2, initializer=thread_init) as pool:
     try:
-        pool.map(thread_execute, paths)
+        clang_results = []
+        for results in pool.map(thread_execute, paths):
+            clang_results.extend(results)
     except KeyboardInterrupt:
         pool.terminate()
         pool.join()