Compile RegEx to improve performance

[rprouse]

@OsirisTerje and I have been working with the Visual Studio test team looking at performance. Our XML serialization in TNode shows up as a bottleneck for performance which isn't really a surprise. One area in there that showed up fairly high was our use of RegEx to escape invalid XML characters. We create this RegEx as a static and use it many times. Adding the compiled option to the RegEx produced a small performance gain, so I did the same for our two other long lasting RegEx's.

This was tested using a test project from the VSTest team with 10,000 empty test methods. Running those tests using the console, in-process, compiled release, took an average of 10.65 sec over 4 runs. With this fix, the average for 4 runs drops to 9.65 sec.

Even though this is nearly a 10% performance gain, it is only for trivial tests and the gain would be smaller if the tests were more complex. It is a minor fix though, so probably worth it.

[mikkelbu]

LGTM

[ChrisMaddock]

Looks good!

Out of interest, did you compare this to using something like XmlConvert.IsXmlChar() over our own regex? It's only post-.NET-4.0, but could be worth using on the platforms where it's available.