Performance improvements in EmailAddressAttribute (dotnet/corefx#36994)

Performance improvements in EmailAddressAttribute (dotnet/corefx#36994)

* performance improvements by a factor of 2 to 10

I get a perf improvement by using IndexOf which ranges from a a factor of two for an email address having 20 char to nearly 10 for 60 char.

* Update src/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs

force explicit type

* Simplify code

Commit migrated from https://github.com/dotnet/corefx/commit/556e080cce3bbd075f7199286b33842f5c0a0c61