Include Host.h for getProcessTriple

https://github.com/llvm/llvm-project/commit/213aea4c5836 means this
header doesn't get implicitly transitively included anymore, so we have
to include it explicitly.

lib/Serialization/SerializedModuleLoader.cpp

@@ -23,6 +23,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Host.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Debug.h"