[NFC] Use full qualified name with std::nullptr_t type (#6737)

This fixes the following compiler error with clang 18.1.6 with mingw-w64
toolchain.
    
microcom.h:190:43: error: unknown type name 'nullptr_t'; did you mean
'std::nullptr_t'?
190 | template <typename T> HRESULT AssignToOut(nullptr_t value, T
*pResult) {
          |                                           ^~~~~~~~~
          |                                           std::nullptr_t
microcom.h:207:43: error: unknown type name 'nullptr_t'; did you mean
'std::nullptr_t'?
207 | template <typename T> void AssignToOutOpt(nullptr_t value, T
*pResult) {
          |                                           ^~~~~~~~~
          |                                           std::nullptr_t

Author: Biswa96

include/dxc/Support/microcom.h

@@ -187,7 +187,7 @@ template <typename T> HRESULT AssignToOut(T value, T *pResult) {
   *pResult = value;
   return S_OK;
 }
-template <typename T> HRESULT AssignToOut(nullptr_t value, T *pResult) {
+template <typename T> HRESULT AssignToOut(std::nullptr_t value, T *pResult) {
   if (pResult == nullptr)
     return E_POINTER;
   *pResult = value;
@@ -204,7 +204,7 @@ template <typename T> void AssignToOutOpt(T value, T *pResult) {
   if (pResult != nullptr)
     *pResult = value;
 }
-template <typename T> void AssignToOutOpt(nullptr_t value, T *pResult) {
+template <typename T> void AssignToOutOpt(std::nullptr_t value, T *pResult) {
   if (pResult != nullptr)
     *pResult = value;
 }