[tools] add -help option for opt. (microsoft#7002)

Add -help option to opt in opt.cpp.

The -help option for opt in clean llvm3.7 was disabled with 
[disable
-help](microsoft@d5bb308#diff-1c7e1b16bc72f52ebd811ef2a24aa91fc4df0f9b47c279b75d0bbb0ae1684d0aR1719)

Fixes microsoft#5514

Author: python3kgae

test/HLSL/opt/help.ll

@@ -0,0 +1,5 @@
+; RUN: opt -help | FileCheck %s
+
+; Make sure the help message is printed.
+; CHECK: -O1
+; CHECK-SAME: - Optimization level 1. Similar to clang -O1

tools/opt/opt.cpp

@@ -128,6 +128,10 @@ StandardLinkOpts("std-link-opts",
                  cl::desc("Include the standard link time optimizations"));
 #endif // HLSL Change Ends
 
+// HLSL Change Starts: add help option.
+static cl::opt<bool> Help("help", cl::desc("Print help"));
+// HLSL Change Ends
+
 static cl::opt<bool>
 OptLevelO1("O1",
            cl::desc("Optimization level 1. Similar to clang -O1"));
@@ -383,6 +387,12 @@ int __cdecl main(int argc, char **argv) {
 
   cl::ParseCommandLineOptions(argc, argv,
     "llvm .bc -> .bc modular optimizer and analysis printer\n");
+  // HLSL Change Starts: add help option.
+  if (Help) {
+    cl::PrintHelpMessage();
+    return 2;
+  }
+  // HLSL Change Ends
 
   if (AnalyzeOnly && NoOutput) {
     errs() << argv[0] << ": analyze mode conflicts with no-output mode.\n";