[lldb] make it easier to find LLDB's python

It is surprisingly difficult to write a simple python script that
can reliably `import lldb` without failing, or crashing.   I'm
currently resorting to convolutions like this:

    def find_lldb(may_reexec=False):
		if prefix := os.environ.get('LLDB_PYTHON_PREFIX'):
			if os.path.realpath(prefix) != os.path.realpath(sys.prefix):
				raise Exception("cannot import lldb.\n"
					f"  sys.prefix should be: {prefix}\n"
					f"  but it is: {sys.prefix}")
		else:
			line1, line2 = subprocess.run(
				['lldb', '-x', '-b', '-o', 'script print(sys.prefix)'],
				encoding='utf8', stdout=subprocess.PIPE,
				check=True).stdout.strip().splitlines()
			assert line1.strip() == '(lldb) script print(sys.prefix)'
			prefix = line2.strip()
			os.environ['LLDB_PYTHON_PREFIX'] = prefix

		if sys.prefix != prefix:
			if not may_reexec:
				raise Exception(
					"cannot import lldb.\n" +
					f"  This python, at {sys.prefix}\n"
					f"  does not math LLDB's python at {prefix}")
			os.environ['LLDB_PYTHON_PREFIX'] = prefix
			python_exe = os.path.join(prefix, 'bin', 'python3')
			os.execl(python_exe, python_exe, *sys.argv)

		lldb_path = subprocess.run(['lldb', '-P'],
			check=True, stdout=subprocess.PIPE,
				encoding='utf8').stdout.strip()

		sys.path = [lldb_path] + sys.path

This patch aims to replace all that with:

  #!/usr/bin/env lldb-python
  import lldb
  ...

... by adding the following features:

* new command line option: --print-script-interpreter-info.  This
   prints language-specific information about the script interpreter
   in JSON format.

* new tool (unix only): lldb-python which finds python and exec's it.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D112973

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -410,6 +410,35 @@ FileSpec ScriptInterpreterPython::GetPythonDir() {
   return g_spec;
 }
 
+StructuredData::DictionarySP ScriptInterpreterPython::GetInterpreterInfo() {
+  GIL gil;
+  FileSpec python_dir_spec = GetPythonDir();
+  if (!python_dir_spec)
+    return nullptr;
+  PythonString python_dir(python_dir_spec.GetPath());
+  PythonDictionary info(PyInitialValue::Empty);
+  llvm::Error error = info.SetItem("lldb-pythonpath", python_dir);
+  if (error)
+    return nullptr;
+  static const char script[] = R"(
+def main(info):
+  import sys
+  import os
+  name = 'python' + str(sys.version_info.major)
+  info.update({
+    "language": "python",
+    "prefix": sys.prefix,
+    "executable": os.path.join(sys.prefix, "bin", name),
+  })
+  return info
+)";
+  PythonScript get_info(script);
+  auto info_json = unwrapIgnoringErrors(As<PythonDictionary>(get_info(info)));
+  if (!info_json)
+    return nullptr;
+  return info_json.CreateStructuredDictionary();
+}
+
 void ScriptInterpreterPython::SharedLibraryDirectoryHelper(
     FileSpec &this_file) {
   // When we're loaded from python, this_file will point to the file inside the