[Performance]: Microsoft.Build.Evaluation.Expander can be more efficient by using Spans

[ToddGrun]

Issue Description

Noticed this while looking into a profile of VS solution load, particularly into string.Substring usage. ExpandPropertiesLeaveTypedAndEscaped showed up in this context, as about 0.3% of all allocations in VS during the solution load.

This led me to ping Rainer with the following questions, and he asked me to file this issue to track.

1) The substring call looks like it's done potentially twice, could the second call just use the existing value if it's not null?
2) The first calls ExpandRegistryValue with the substring value. It looks like that method could just take in a ReadOnlySpan<char> and use that.
3) The second calls ExpandPropertyBody with the substring value. This one has a lot of uses of that value, but again, it appears it might be ReadOnlySpan'able.
 
Also, there is the SplitFileSpec call which is a separate question. I assume that changing that to ReadOnlySpans isn't feasible?

Steps to Reproduce

Open Roslyn.sln in VS

Data

The etl failed uploading several times. I've copied to an internal share: \vwdbuild01\dev\toddgrun\msbuild\PerfViewData.etl.zip

Analysis

No response

Versions & Configurations

VS 17.13

Regression

• yes
• no

Regression Details

No response