Remove platform macros from tip_text_service.cc

This commit removes the dependency on platform macros such as _M_X64 and
_M_IX86 from tip_text_service.cc.

There must be no change in the final artifacts.

This is a preparation to build Mozc for ARM64 (google#1130).

src/win32/tip/tip_text_service.cc

@@ -36,8 +36,10 @@
 #include <wil/com.h>
 #include <windows.h>
 
+#include <bit>
 #include <cstddef>
 #include <cstdint>
+#include <functional>
 #include <memory>
 #include <string>
 #include <utility>
@@ -249,15 +251,11 @@ wil::com_ptr_nothrow<ITfCategoryMgr> GetCategoryMgr() {
 template <typename T>
 struct ComPtrHash {
   size_t operator()(const wil::com_ptr_nothrow<T> &value) const {
-    // Caveats: On x86 environment, both _M_X64 and _M_IX86 are defined. So we
-    //     need to check _M_X64 first.
-#if defined(_M_X64)
-    constexpr size_t kUnusedBits = 3;  // assuming 8-byte aligned
-#elif defined(_M_IX86)                 // defined(_M_X64)
-    constexpr size_t kUnusedBits = 2;  // assuming 4-byte aligned
-#else                                  // defined(_M_IX86)
-#error "unsupported platform"
-#endif  // defined(_M_IX86)
+    // The minimum size of COM objects is the pointer to vtable.
+    // For instance the last 3 bits are guaranteed to be zero on 64-bit
+    // processes.
+    constexpr size_t kUnusedBits =
+        std::max(std::bit_width(sizeof(void *)), 1) - 1;
     // Compress the data by shifting unused bits.
     return reinterpret_cast<size_t>(value.get()) >> kUnusedBits;
   }