Improve SyntaxRewriter performance for operator folding

[ahoppen]

Running operator folding on MovieSwiftUI takes as long as parsing. That really shouldn’t be the case. Running with Instruments shows that we spend the majority of time in memory-related operations.

The first steps to take would be:

• Add an arena parameter to all the syntax node initializers
• Add an arena parameter to SyntaxRewriter
• Construct all nodes in SyntaxRewriter and operator folding in that arena
• Investigate where else SyntaxRewriter is causing memory allocations and try to eliminate them

Attached is a WIP diff where I started to eliminate some of the memory allocations by SyntaxRewriter but I stopped once I realized that we’d need to modify the initializers of all syntax nodes.

reduce-arena-memory-allocations.txt