lit: Move RUN at line comment after the command.

When a developer copy/pastes a failing command line into their
shell to rerun it, they have to manually delete the "RUN: at line
N:" prefix. To make life easier for such developers, let's make it
possible to copy/paste a command without needing to modify it while
still showing the line number in the output by moving the line number
to a comment at the end of the command line.

Reviewers: jroelofs, MaskRay

Reviewed By: jroelofs, MaskRay

Pull Request: llvm#132485

Author: pcc

llvm/docs/CommandGuide/lit.rst

@@ -100,7 +100,7 @@ OUTPUT OPTIONS
 
  Each command is printed before it is executed. This can be valuable for
  debugging test failures, as the last printed command is the one that failed.
- Moreover, :program:`lit` inserts ``'RUN: at line N'`` before each
+ Moreover, :program:`lit` inserts ``'RUN: at line N'`` after each
  command pipeline in the output to help you locate the source line of
  the failed command.
 

llvm/utils/lit/lit/TestRunner.py

@@ -1231,7 +1231,7 @@ def executeScript(test, litConfig, tmpBase, commands, cwd):
                 # the shell's execution trace for the 'set' commands by
                 # redirecting their stderr to /dev/null.
                 if command:
-                    msg = f"'{dbg}': {shlex.quote(command.lstrip())}"
+                    msg = f"{shlex.quote(command.lstrip())} \\# '{dbg}'"
                 else:
                     msg = f"'{dbg}' has no command after substitutions"
                 commands[i] = (