ExpandPropertiesLeaveTypedAndEscaped (incorrectly) makes the assumption that it's ReuseableStringBuilder won't be resized

[davkean]

Opening a large 1000 project solution based on a solution with http://github.com/dotnet/project-system's project duplicated many times, I can see that 8.4% of all allocations during solution load (before we OOM) is ExpandPropertiesLeaveTypedAndEscaped expanding its ReuseableStringBuilder:

This seems to indicate that this is not required:

                    // This method is called very often - of the order of 3,000 times per project.
                    // With the reuseable string builder, there's no particular need to initialize the length as it will already have grown.
                    using (var result = new ReuseableStringBuilder())

I suspect we're not caching the builder because it's grown too big so the comment will be incorrect. We should set a sensible initial size based on what we're going to put inside it.

[ladipro]

This was optimized by using SpanBasedConcatenator instead of ReuseableStringBuilder in #6128.