Avoid allocating in TimeZoneInfo.GetHashCode() (dotnet/coreclr#8513)

Avoid the intermediate ToUpper string allocation.

Commit migrated from dotnet/coreclr@bf4ed11

src/coreclr/src/mscorlib/src/System/TimeZoneInfo.cs

@@ -969,7 +969,7 @@ static public TimeZoneInfo FromSerializedString(string source) {
         // GetHashCode -
         //
         public override int GetHashCode() {
-            return m_id.ToUpper(CultureInfo.InvariantCulture).GetHashCode();
+            return StringComparer.OrdinalIgnoreCase.GetHashCode(m_id);
         }
 
         //