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

[fredemmott]

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.