[WDF] Add Windows Driver Framework files

Takern from Microsoft GitHub repo:
d9c6040fe9

Licensed under MIT

sdk/lib/drivers/wdf/shared/support/fxstring.cpp

@@ -0,0 +1,67 @@
+/*++
+
+Copyright (c) Microsoft Corporation
+
+Module Name:
+
+    FxString.cpp
+
+Abstract:
+
+    This module implements a simple string class to operate on
+    unicode strings.
+
+Author:
+
+
+
+Environment:
+
+    Both kernel and user mode
+
+Revision History:
+
+--*/
+
+#include "FxSupportPch.hpp"
+
+FxString::FxString(
+    __in PFX_DRIVER_GLOBALS FxDriverGlobals
+    ) :
+    FxObject(FX_TYPE_STRING, sizeof(FxString), FxDriverGlobals)
+{
+    RtlInitUnicodeString(&m_UnicodeString, NULL);
+    MarkPassiveDispose(ObjectDoNotLock);
+}
+
+FxString::~FxString()
+{
+    if (m_UnicodeString.Buffer) {
+        FxPoolFree(m_UnicodeString.Buffer);
+    }
+}
+
+_Must_inspect_result_
+NTSTATUS
+FxString::Assign(
+    __in const UNICODE_STRING* UnicodeString
+    )
+{
+    return FxDuplicateUnicodeString(GetDriverGlobals(),
+                                    UnicodeString,
+                                    &m_UnicodeString);
+}
+
+_Must_inspect_result_
+NTSTATUS
+FxString::Assign(
+    __in PCWSTR SourceString
+    )
+
+{
+    UNICODE_STRING string;
+
+    RtlInitUnicodeString(&string, SourceString);
+
+    return Assign(&string);
+}