Avoid a race condition in opt-viewer/optrecord

See https://bugzilla.redhat.com/2336915
See https://reviews.llvm.org/D41784?id=
See androm3da/optviewer-demo#4 (comment)

Fixes #62403

The race condition happened when the demangler_proc was being set.
The locking mechanism itself happened too late.

This way, the lock always exists (to avoid a race when creating it)
and is always used when *creating* demangler_proc.

I don't have any prior experience with the codebase
and I cannot guarantee the behavior is correct.

It appears to get rid of the race (which I was seeing consistently).

    multiprocessing.pool.RemoteTraceback:
    """
    Traceback (most recent call last):
      File "/usr/lib64/python3.14/multiprocessing/pool.py", line 125, in worker
        result = (True, func(*args, **kwds))
                        ~~~~^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.14/multiprocessing/pool.py", line 48, in mapstar
        return list(map(*args))
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/optpmap.py", line 25, in _wrapped_func
        return func(argument, filter_)
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/opt-viewer.py", line 293, in _render_file
        SourceFileRenderer(source_dir, output_dir, filename, no_highlight).render(remarks)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/opt-viewer.py", line 216, in render
        self.render_source_lines(self.source_stream, line_remarks)
        ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/opt-viewer.py", line 123, in render_source_lines
        self.render_inline_remarks(remark, html_line)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/opt-viewer.py", line 126, in render_inline_remarks
        inlining_context = r.DemangledFunctionName
                           ^^^^^^^^^^^^^^^^^^^^^^^
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/optrecord.py", line 153, in DemangledFunctionName
        return self.demangle(self.Function)
               ~~~~~~~~~~~~~^^^^^^^^^^^^^^^
      File "/builddir/build/BUILD/llvm-19.1.6-build/llvm-project-19.1.6.src/llvm/tools/opt-viewer/optrecord.py", line 77, in demangle
        with cls.demangler_lock:
             ^^^^^^^^^^^^^^^^^^
    AttributeError: type object 'Passed' has no attribute 'demangler_lock'. Did you mean: 'demangler_proc'?
    """

Author: hroncok

llvm/tools/opt-viewer/optrecord.py

@@ -64,17 +64,19 @@ class Remark(yaml.YAMLObject):
 
     default_demangler = "c++filt -n"
     demangler_proc = None
+    demangler_lock = Lock()
 
     @classmethod
     def set_demangler(cls, demangler):
         cls.demangler_proc = subprocess.Popen(
             demangler.split(), stdin=subprocess.PIPE, stdout=subprocess.PIPE
         )
-        cls.demangler_lock = Lock()
 
     @classmethod
     def demangle(cls, name):
         with cls.demangler_lock:
+            if not cls.demangler_proc:
+                cls.set_demangler(cls.default_demangler)
             cls.demangler_proc.stdin.write((name + "\n").encode("utf-8"))
             cls.demangler_proc.stdin.flush()
             return cls.demangler_proc.stdout.readline().rstrip().decode("utf-8")
@@ -323,8 +325,6 @@ def get_remarks(input_file, filter_=None):
 def gather_results(filenames, num_jobs, should_print_progress, filter_=None):
     if should_print_progress:
         print("Reading YAML files...")
-    if not Remark.demangler_proc:
-        Remark.set_demangler(Remark.default_demangler)
     remarks = optpmap.pmap(
         get_remarks, filenames, num_jobs, should_print_progress, filter_
     )