[CoreFoundation] Use a Windows Timezone if we are given one

Before attempting to check if a timezone is an abbreviation or Olson
format, we should make sure we can't just query Windows for the
timezone. For example, CFTimeZoneCopyKnownNames will return UTC[-+XX]
timezones which we would otherwise would fail on.

Author: gmittert

CoreFoundation/NumberDate.subproj/CFTimeZone.c

@@ -1307,6 +1307,15 @@ Boolean _CFTimeZoneInit(CFTimeZoneRef timeZone, CFStringRef name, CFDataRef data
     Boolean result = false;
 
 #if TARGET_OS_WIN32
+    // Start by checking if we're just given a timezone Windows knows about
+    int32_t offset;
+    __CFTimeZoneGetOffset(name, &offset);
+    if (offset) {
+        // TODO: handle DST
+        __CFTimeZoneInitFixed(timeZone, offset, name, 0);
+        return TRUE;
+    }
+
     CFDictionaryRef abbrevs = CFTimeZoneCopyAbbreviationDictionary();
 
     tzName = CFDictionaryGetValue(abbrevs, name);
@@ -1319,9 +1328,8 @@ Boolean _CFTimeZoneInit(CFTimeZoneRef timeZone, CFStringRef name, CFDataRef data
     CFRelease(abbrevs);
 
     if (tzName) {
-        int32_t offset;
         __CFTimeZoneGetOffset(tzName, &offset);
-        // TODO(compnerd) handle DST
+        // TODO: handle DST
         __CFTimeZoneInitFixed(timeZone, offset, name, 0);
         return TRUE;
     }