Don't include <iostream> in MetadataSource.h because it's not actually needed (#39845)

Author: kubamracek

include/swift/Reflection/MetadataSource.h

@@ -28,7 +28,7 @@
 using llvm::cast;
 
 #include <climits>
-#include <iostream>
+#include <string>
 
 namespace swift {
 namespace reflection {

tools/swift-reflection-dump/swift-reflection-dump.cpp

@@ -41,6 +41,7 @@
 
 #include <algorithm>
 #include <csignal>
+#include <iostream>
 
 using llvm::ArrayRef;
 using llvm::dyn_cast;