Avoid keeping the SourceText instance around in SyntaxTree

[alexrp]

celerity/src/language/core/Syntax/SyntaxAnalysis.cs

Lines 6 to 9 in 18c5d49

	public sealed class SyntaxAnalysis
	{
	// TODO: We should eventually get rid of this. When we need the source text, we can reconstruct it from the tree.
	public SourceText Text { get; }

This is neat because, for the happy case, we don't need to access the SourceText at all. Only when there are diagnostics do we need to access line information from the SourceText, and it's reasonable to just reconstruct it for those cases.

Even then, we still have to keep the SourceText around during parsing in order to construct locations for diagnostics. To remedy that, we should probably also consider changing SourceDiagnostic to not carry a SourceLocation, but rather a SourceTextSpan and a SyntaxItem reference. A SourceDiagnostic.GetLocation() method could then be exposed which would resolve the SourceLocation.

[alexrp]

The diagnostic aspect has been handled, along with some related refactoring/cleanup.

At this point, I think the Text property can be turned into a GetText() method that returns the existing SourceText (if alive) or reconstructs it from the tree. It would be nice to keep an existing SourceText around as something like a SoftReference<T> so that it's not constantly recomputed, but can also still be collected if the GC is pressured.