[dexter] Add keyword argument 'on_line' to DexLabel

[Orlando]

Summary

Add optional keyword argument 'on_line' to DexLabel to label the specifed line
instead of the line the command is found on.

This will be helpful when used alongside DexDeclareFile (D99651).

[Orlando]

+ Update Commands.md

[TWeaver]

Nice! LGTM

debuginfo-tests/dexter/dex/command/commands/DexLabel.py

 # DExTer : Debugging Experience Tester
 # ~~~~~~   ~         ~~         ~   ~~
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 """Command used to give a line in a test a named psuedonym. Every DexLabel has
    a line number and Label string component.
 """
 
 from dex.command.CommandBase import CommandBase
 
 
 class DexLabel(CommandBase):
-    def __init__(self, label):
+    def __init__(self, label, **kwargs):
 
         if not isinstance(label, str):
             raise TypeError('invalid argument type')
 
+        try:
+            self.on_line = kwargs.pop('on_line')
+        except KeyError:
+            # We cannot use self.lineno because it hasn't been set yet.
+            pass
+        if kwargs:
+            raise TypeError(f'unexpected named args: {", ".join(kwargs)}')
+
         self._label = label
         super(DexLabel, self).__init__()
 
+    def get_line(self):
+        return getattr(self, 'on_line', self.lineno)
+
     def get_as_pair(self):
-        return (self._label, self.lineno)
+        return (self._label, self.get_line())
 
     @staticmethod
     def get_name():
         return __class__.__name__
 
     def eval(self):
         return self._label

debuginfo-tests/dexter/feature_tests/subtools/test/err_label_kwarg.cpp

+// Purpose:
+//    Check that bad keyword args in \DexLabel are reported.
+//    Use --binary switch to trick dexter into skipping the build step.
+//
+// RUN: not %dexter_base test --binary %s --debugger 'lldb' -- %s | FileCheck %s
+// CHECK: parser error:{{.*}}err_label_kwarg.cpp(8): unexpected named args: bad_arg
+
+// DexLabel('test', bad_arg=0)

debuginfo-tests/dexter/feature_tests/subtools/test/label_another_line.cpp

+// Purpose:
+//    Check that the optional keyword argument 'on_line' makes a \DexLabel label
+//    that line instead of the line the command is found on.
+//
+// RUN: %dexter_regression_test -- %s | FileCheck %s
+// CHECK: label_another_line.cpp: (1.0000)
+
+int main() {
+  int result = 0;
+  return result;
+}
+
+// DexLabel('test', on_line=10)
+// DexExpectWatchValue('result', '0', on_line='test')