Revert and patch "[Python] Remove readline module"

Fix https://bugs.llvm.org/show_bug.cgi?id=43830 while avoiding polluting the
global Python namespace.

This both reverts r357277 to rebundle a version of Python's readline module
based on libedit.

However, this patch also provides two improvements over the previous
implementation:

1. use PyMem_RawMalloc instead of PyMem_Malloc, as expected by PyOS_Readline
   (prevents to segfault upon exit of interactive session)
2. patch the readline module upon embedded interpreter loading, instead of
   patching it globally, which should prevent any side effect on other
   modules/packages
3. only activate the patched module if libedit is actually linked in lldb

Differential Revision: https://reviews.llvm.org/D69793

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

@@ -16,6 +16,7 @@
 #include "lldb-python.h"
 
 #include "PythonDataObjects.h"
+#include "PythonReadline.h"
 #include "ScriptInterpreterPythonImpl.h"
 
 #include "lldb/API/SBFrame.h"
@@ -217,6 +218,22 @@ public:
 
     InitializePythonHome();
 
+#ifdef LLDB_USE_LIBEDIT_READLINE_COMPAT_MODULE
+    // Python's readline is incompatible with libedit being linked into lldb.
+    // Provide a patched version local to the embedded interpreter.
+    bool ReadlinePatched = false;
+    for (auto *p = PyImport_Inittab; p->name != NULL; p++) {
+      if (strcmp(p->name, "readline") == 0) {
+        p->initfunc = initlldb_readline;
+        break;
+      }
+    }
+    if (!ReadlinePatched) {
+      PyImport_AppendInittab("readline", initlldb_readline);
+      ReadlinePatched = true;
+    }
+#endif
+
     // Register _lldb as a built-in module.
     PyImport_AppendInittab("_lldb", LLDBSwigPyInit);