Avoid #include <sstream> when not needed (#40174)

Author: kubamracek

stdlib/public/Reflection/MetadataSource.cpp

@@ -12,8 +12,6 @@
 
 #include "swift/Reflection/MetadataSource.h"
 
-#include <sstream>
-
 using namespace swift;
 using namespace reflection;
 

stdlib/public/stubs/Stubs.cpp

@@ -42,7 +42,9 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#if defined(__CYGWIN__) || defined(__HAIKU__)
 #include <sstream>
+#endif
 #if defined(__OpenBSD__) || defined(__ANDROID__) || defined(__linux__) || defined(__wasi__) || defined(_WIN32)
 #include <locale.h>
 #if defined(_WIN32)
@@ -288,6 +290,7 @@ static bool swift_stringIsSignalingNaN(const char *nptr) {
   return false;
 }
 
+#if defined(__CYGWIN__) || defined(__HAIKU__)
 // This implementation should only be used on platforms without the
 // relevant strto* functions, such as Cygwin or Haiku.
 // Note that using this currently causes test failures.
@@ -308,6 +311,7 @@ T _swift_strto(const char *nptr, char **endptr) {
 
   return ParsedValue;
 }
+#endif
 
 #if defined(__OpenBSD__) || defined(_WIN32) || defined(__CYGWIN__) || defined(__HAIKU__)
 #define NEED_SWIFT_STRTOD_L