Add missing headers for clang with libc++ builds (#5285)

Add these headers to successfully build DXC with clang and libc++.

lib/DxilDia/DxcPixBase.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "dxc/Support/WinIncludes.h"
+#include <stdexcept>
 
 namespace dxil_debug_info
 {

lib/Support/raw_ostream.cpp

@@ -28,6 +28,7 @@
 #include <ios>
 #include <sys/stat.h>
 #include <system_error>
+#include <float.h> // HLSL Change: add for '_fpclass'
 
 // <fcntl.h> may provide O_BINARY.
 #if defined(HAVE_FCNTL_H)