Reimplement GetWindowsLocalTimeZone without icu.h

This commit reimplements my previous commits [1][2], which removed the
dependency on WinRT on Windows by using system "icu.dll" (#315) when
<icu.h> is available in the build environment.

Here are major improvements:

 * The relevant code is extracted into separate source files
   "src/time_zone_name_win.{cc,h}". This is also a preparation for
   implementing TimeZoneIf with Windows time APIs (#328), where one more
   ICU API needs to be dynamically imported.

 * The dependency on <icu.h> is removed by locally defining required
   constants and API prototypes. This should make it possible to build
   CCTZ with the MinGW toolchain, where <icu.h> is not yet available.

 * LoadLibraryExW(L"icu.dll", nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32) is
   replaced with LoadLibraryW with the full path to "icu.dll" in the
   System32 directory, because LOAD_LIBRARY_SEARCH_SYSTEM32 can be
   ignored when "icu.dll" is already loaded from a non-System32
   directory. Let's stick to the system "icu.dll" in favor of
   predictability.

Other than the above improvements, the overall logic remains unchanged.

 [1]: 2ebbe0d
 [2]: f8e494f

Author: yukawa

src/time_zone_name_win.cc

@@ -30,8 +30,8 @@
 namespace cctz {
 namespace {
 
-// UChar is defined as char16_t in ICU by default, but it is also safe to assume
-// wchar_t and char16_t are equivalent on Windows.
+// Define UChar as wchar_t here because Win32 APIs receive UTF-16 strings as
+// wchar_t* instead of char16_t*. Using char16_t would require additional casts.
 using UChar = wchar_t;
 
 enum UErrorCode : std::int32_t {
@@ -49,8 +49,7 @@ std::atomic<bool> g_unavailable;
 std::atomic<ucal_getTimeZoneIDForWindowsID_func>
     g_ucal_getTimeZoneIDForWindowsID;
 
-template <typename T>
-static T AsProcAddress(HMODULE module, const char* name) {
+template <typename T> static T AsProcAddress(HMODULE module, const char* name) {
   static_assert(
       std::is_pointer<T>::value &&
           std::is_function<typename std::remove_pointer<T>::type>::value,
@@ -61,12 +60,11 @@ static T AsProcAddress(HMODULE module, const char* name) {
 
 std::wstring GetSystem32Dir() {
   std::wstring result;
-  // TODO: Remove std::max() when we require C++17 or higher.
-  auto len = std::max<size_t>(result.capacity(), 1);
+  std::uint32_t len = static_cast<std::uint32_t>(std::min<size_t>(
+      result.capacity(), std::numeric_limits<std::uint32_t>::max()));
   do {
     result.resize(len);
-    // TODO: Switch to std::string::data() when we require C++17 or higher.
-    len = ::GetSystemDirectoryW(&result[0], static_cast<UINT>(result.size()));
+    len = ::GetSystemDirectoryW(&result[0], len);
   } while (len > result.size());
   result.resize(len);
   return result;
@@ -129,14 +127,13 @@ std::string Utf16ToUtf8(const wchar_t* ptr, size_t size) {
   }
   const int chars_len = static_cast<int>(size);
   std::string result;
-  // TODO: Remove std::max() when we require C++17 or higher.
-  auto len = std::max<size_t>(result.capacity(), 1);
+  std::int32_t len = static_cast<std::int32_t>(std::min<size_t>(
+      result.capacity(), std::numeric_limits<std::int32_t>::max()));
   do {
     result.resize(len);
     // TODO: Switch to std::string::data() when we require C++17 or higher.
     len = ::WideCharToMultiByte(CP_UTF8, WC_ERR_INVALID_CHARS, ptr, chars_len,
-                                &result[0], static_cast<int>(result.size()),
-                                nullptr, nullptr);
+                                &result[0], len, nullptr, nullptr);
   } while (len > result.size());
   result.resize(len);
   return result;
@@ -156,15 +153,13 @@ std::string GetWindowsLocalTimeZone() {
   }
 
   std::wstring result;
-  // TODO: Remove std::max() when we require C++17 or higher.
-  auto len = std::max<size_t>(result.capacity(), 1);
-  while (true) {
+  std::int32_t len = static_cast<std::int32_t>(std::min<size_t>(
+      result.capacity(), std::numeric_limits<std::int32_t>::max()));
+  for (;;) {
     UErrorCode status = U_ZERO_ERROR;
     result.resize(len);
-    // TODO: Switch to std::string::data() when we require C++17 or higher.
     len = getTimeZoneIDForWindowsID(
-        info.TimeZoneKeyName, -1, nullptr, &result[0],
-        static_cast<std::int32_t>(result.size()), &status);
+        info.TimeZoneKeyName, -1, nullptr, &result[0], len, &status);
     if (U_SUCCESS(status)) {
       return Utf16ToUtf8(result.data(), len);
     }