[lit] support dxopt in lit (#5515)

When input is from stdin, need to add '\n' after each line.

This is for use dxopt to test pix passes.
opt doesn't support passes which has parameters right now. We could
switch it to opt once pass parameter is supported.

tools/clang/test/HLSLFileCheck/pix/DebugAs.hlsl → tools/clang/test/PIX/DebugAs.hlsl

@@ -1,4 +1,4 @@
-// RUN: %dxc -Emain -Tas_6_5 %s | %opt -S -hlsl-dxil-debug-instrumentation,parameter0=10,parameter1=20,parameter2=30 | %FileCheck %s
+// RUN: %dxc -Emain -Tas_6_5 %s | %dxopt - -S -hlsl-dxil-debug-instrumentation,parameter0=10,parameter1=20,parameter2=30 | FileCheck %s
 
 // Check that the AS thread IDs are added properly
 

tools/clang/tools/dxopt/dxopt.cpp

@@ -69,15 +69,15 @@ static HRESULT ReadStdin(std::string &input) {
     std::getline(std::cin, line);
     if (line.empty()) {
       emptyLine = true;
-      break;
+      continue;
     }
 
     std::copy(line.begin(), line.end(),
               std::back_inserter(input));
+    input.push_back('\n');
   }
 
   DWORD lastError = GetLastError();
-
   // Make sure we reached finished successfully.
   if (std::cin.eof())
     return S_OK;