Instead of creating the AST directly in the parser (and libSyntax or
SwiftSyntax via SyntaxParsingContext), make Parser to explicitly create
a tree of ParsedSyntaxNodes. Their OpaqueSyntaxNodes can be either
libSyntax or SwiftSyntax. If AST is needed, it can be generated from the
libSyntax tree.
This eliminates the overhead of ParsedRawSyntaxNode needing to do memory management.
If ParsedRawSyntaxNode needs to point to some data the memory is allocated from a bump allocator.
There are also some improvements on how the ParsedSyntaxBuilders work.
Instead of creating syntax nodes directly, modify the parser to invoke an abstract interface 'SyntaxParseActions' while it is parsing the source code.
This decouples the act of parsing from the act of forming a syntax tree representation.
'SyntaxTreeCreator' is an implementation of SyntaxParseActions that handles the logic of creating a syntax tree.
To enforce the layering separation of parsing and syntax tree creation, a static library swiftSyntaxParse is introduced to compose the two.
This decoupling is important for introducing a syntax parser library for SwiftSyntax to directly access parsing.
