[lldb] Accept negative indexes in __getitem__

To the Python bindings, add support for Python-like negative indexes.

While was using `script`, I tried to access a thread's bottom frame with
`thread.frame[-1]`, but that failed. This change updates the `__getitem__`
implementations to support negative indexes as one would expect in Python.

Differential Revision: https://reviews.llvm.org/D143282

lldb/test/API/python_api/breakpoint/TestBreakpointAPI.py

@@ -62,6 +62,9 @@ class BreakpointAPITestCase(TestBase):
         location = breakpoint.GetLocationAtIndex(0)
         self.assertTrue(location.IsValid())
 
+        # Test negative index access.
+        self.assertTrue(breakpoint.location[-1].IsValid())
+
         # Make sure the breakpoint's target is right:
         self.assertEqual(target, breakpoint.GetTarget(), "Breakpoint reports its target correctly")
         

lldb/test/API/python_api/thread/TestThreadAPI.py

@@ -48,6 +48,11 @@ class ThreadAPITestCase(TestBase):
         self.setTearDownCleanup(dictionary=d)
         self.step_over_3_times(self.exe_name)
 
+    def test_negative_indexing(self):
+        """Test SBThread.frame with negative indexes."""
+        self.build()
+        self.validate_negative_indexing()
+
     def setUp(self):
         # Call super's setUp().
         TestBase.setUp(self)
@@ -269,3 +274,29 @@ class ThreadAPITestCase(TestBase):
         thread.RunToAddress(start_addr)
         self.runCmd("process status")
         #self.runCmd("thread backtrace")
+
+    def validate_negative_indexing(self):
+        exe = self.getBuildArtifact("a.out")
+
+        target = self.dbg.CreateTarget(exe)
+        self.assertTrue(target, VALID_TARGET)
+
+        breakpoint = target.BreakpointCreateByLocation(
+            "main.cpp", self.break_line)
+        self.assertTrue(breakpoint, VALID_BREAKPOINT)
+        self.runCmd("breakpoint list")
+
+        # Launch the process, and do not stop at the entry point.
+        process = target.LaunchSimple(
+            None, None, self.get_process_working_directory())
+
+        thread = get_stopped_thread(process, lldb.eStopReasonBreakpoint)
+        self.assertTrue(
+            thread.IsValid(),
+            "There should be a thread stopped due to breakpoint")
+        self.runCmd("process status")
+
+        pos_range = range(thread.num_frames)
+        neg_range = range(thread.num_frames, 0, -1)
+        for pos, neg in zip(pos_range, neg_range):
+            self.assertEqual(thread.frame[pos].idx, thread.frame[-neg].idx)