[Bundler] Use argv[0] as the default choice for the Executable name.

[tra]

Summary

The path produced by getMainExecutable() may not be the right one when the files are installed in
a symlinked tree and when the real location of llvm-objdump is in a different directory.

Given that clang-offload-bundler is invoked by clang, the driver already does the job figuring out
the right path (e.g. it pays attention to -no-canonical-prefixes).
Offload bundler should use it, instead of trying to figure out the path on its own.

[tra]

ping?

[sfantao]

@tra, thank you for the patch. This makes sense.

The patch looks good but it has to be someone else to okay it.

[yaxunl]

LGTM. Thanks.

clang/tools/clang-offload-bundler/ClangOffloadBundler.cpp

reportError(createStringError(errc::invalid_argument,
                                       Twine(HostTargetNum)));
   }
 
   if (Error)
     return 1;
 
   // Save the current executable directory as it will be useful to find other
   // tools.
+  BundlerExecutable = argv[0];
+  if (!llvm::sys::fs::exists(BundlerExecutable))
     BundlerExecutable = sys::fs::getMainExecutable(argv[0], &BundlerExecutable);
 
   if (llvm::Error Err = Unbundle ? UnbundleFiles() : BundleFiles()) {
     reportError(std::move(Err));
     return 1;
   }
   return 0;
 }