Performance: reduce overhead of computing full path for items from globs

Based on investigating performance trace to load Roslyn.sln, I looked into the time spent in CPS's UpdateSnapshotCoreAsync, almost 30% thread time in this function is to calculate FullPath of project items.

Because those items are coming from globs, I wonder the path is naturally normalized, and we can reduce some of the overhead here.

```
Name Inc % Inc Inc Ct
microsoft.build.ni!FileUtilities.GetFullPath 28.1 405.567 592
+ microsoft.build.ni!FileUtilities.NormalizePath 19.3 277.811 267
+ system.ni!System.Text.RegularExpressions.Regex.Match(System.String) 5.9 84.554 209
+ mscorlib.ni!System.IO.Path.Combine(System.String, System.String) 1.5 22.189 62
```