llvm-foreach: print full command when failing

[crystarm]

Fixes #20747

Summary

llvm-foreach currently reports generic errors (for example, No such file or directory) without showing which command failed.
This change improves diagnostics by printing the full command line that llvm-foreach attempted to run.

What changed

• Added command formatting helper using sys::printArg(...) to preserve proper quoting.
• Added unified failure reporting that includes the full command line.
• Updated failure handling to print command line for:
  • command lookup failure (findProgramByName),
  • process launch failure (ExecuteNoWait with ExecutionFailed),
  • launched process non-zero exit status (both polling path and final wait path).
• Stored submitted jobs as process + formatted command so failures can always report the exact command.

Tests

Extended llvm-foreach tests on both platforms:

• llvm/test/tools/llvm-foreach/llvm-foreach-lin.ll
• llvm/test/tools/llvm-foreach/llvm-foreach-win.ll

Added checks for:

• non-zero command exit (false / cmd /c exit 1),
• missing executable (llvm-foreach-command-does-not-exist),
• expected llvm-foreach: command: ... output.

Validation

• llvm-lit run for llvm/test/tools/llvm-foreach in local Linux setup (Linux tests pass; Windows-specific test is unsupported as expected).
• Formatting/diagnostics checks for modified files are clean.

[crystarm]

Hi @sarnex!! (。・ω・。)

Could you please take a look when you have a chance?

I'm not entirely sure who would be the best person to ping here... But I'm tagging you since you reported the original issue.

[sarnex]

Maybe @YuriPlyakhin or @maksimsab can take a first look?