Use custom match in FileUtilities.GetFullPath

[rainersigwald]

@davkean, @benvillalobos and I were doing some exploratory PerfView analysis and noticed this.

#3969 tried to reduce the allocation of Regex.Match objects that aren't necessary because all we care about is IsMatch. That simplified the code greatly. But unfortunately, it still allocates a Match object under the covers.

We should replace usages of FileUtilitiesRegex.DrivePattern, FileUtilitiesRegex.UncPattern, StartWithDrivePattern, and StartsWithUncPattern with functions that do the match explicitly without allocation. The patterns are pretty straightforward, so the matcher methods should still be pretty readable.

[rainersigwald]

This will help with part of #2645.

[benvillalobos]

This was fixed in #4304