Make smart pointer constructors (e.g. `wil::com_ptr`) constexpr when initializing to nullptr

e.g. something along the lines of:

diff --git a/include/wil/com.h b/include/wil/com.h
index 2492611..33e126c 100644
--- a/include/wil/com.h
+++ b/include/wil/com.h
@@ -215,12 +215,12 @@ public:
     //! @{
 
     //! Default constructor (holds nullptr).
-    com_ptr_t() WI_NOEXCEPT : m_ptr(nullptr)
+    __WI_LIBCPP_CONSTEXPR com_ptr_t() WI_NOEXCEPT : m_ptr(nullptr)
     {
     }
 
     //! Implicit construction from nullptr_t (holds nullptr).
-    com_ptr_t(wistd::nullptr_t) WI_NOEXCEPT : com_ptr_t()
+    __WI_LIBCPP_CONSTEXPR com_ptr_t(wistd::nullptr_t) WI_NOEXCEPT : com_ptr_t()
     {
     }
 
@@ -234,7 +234,7 @@ public:
     }
 
     //! Copy-construction from a like `com_ptr_t` (copies and AddRef's the parameter).
-    com_ptr_t(const com_ptr_t& other) WI_NOEXCEPT : com_ptr_t(other.get())
+    __WI_LIBCPP_CONSTEXPR com_ptr_t(const com_ptr_t& other) WI_NOEXCEPT : com_ptr_t(other.get())
     {
     }
 
@@ -269,7 +269,7 @@ public:
     //! @{
 
     //! Assign to nullptr (releases the current pointer, holds nullptr).
-    com_ptr_t& operator=(wistd::nullptr_t) WI_NOEXCEPT
+    __WI_LIBCPP_CONSTEXPR com_ptr_t& operator=(wistd::nullptr_t) WI_NOEXCEPT
     {
         reset();
         return *this;
@@ -292,7 +292,7 @@ public:
     }
 
     //! Assign a like `com_ptr_t` (releases current pointer, copies and AddRef's the parameter).
-    com_ptr_t& operator=(const com_ptr_t& other) WI_NOEXCEPT
+    __WI_LIBCPP_CONSTEXPR com_ptr_t& operator=(const com_ptr_t& other) WI_NOEXCEPT
     {
         return operator=(other.get());
     }

Not sending a pull request due to build dependencies on nuget, separately installed detours etc instead of more isolated and common tools like vcpkg.

Start in include/wil/com.h and review the wil::com_ptr constructors and assignments shown in the issue, then identify whether the same nullptr initialization pattern applies to other smart pointer types. Check the available project build requirements before validating constexpr initialization; done means the relevant constructors support nullptr initialization without breaking existing compatibility.