[llvm-lto2] Added version command for llvm-lto2 (#148866)

Previously, the only way to check version for llvm-lto2 was to add
version as a subcommand: `llvm-lto2 run --version`. This adds version as
a main command for llvm-lto2 for more intuitive access.

You can now check version with the command `llvm-lto2 --version`.

Author: fnriv

llvm/test/tools/llvm-lto2/version.test

@@ -0,0 +1,7 @@
+## Show that you can run version as a main command for llvm-lto2 
+## or a subcommand of llvm-lto2 run.
+
+RUN: llvm-lto2 --version | FileCheck %s
+RUN: llvm-lto2 run --version | FileCheck %s 
+
+CHECK: version

llvm/tools/llvm-lto2/llvm-lto2.cpp

@@ -616,5 +616,9 @@ int main(int argc, char **argv) {
     outs() << GlobalValue::getGUIDAssumingExternalLinkage(argv[2]) << '\n';
     return 0;
   }
+  if (Subcommand == "--version") {
+    cl::PrintVersionMessage();
+    return 0;
+  }
   return usage();
 }