Do the same thing that we are already doing for trivia: Since RawSyntax
nodes always live inside a SyntaxArena, we don't need to tail-allocate
an OwnedString to store the token's text. Instead we can just copy it
to the SyntaxArena. If we copy the entire source buffer to the syntax
arena at the start of parsing, this means that no more copies are
required later on. Plus we also avoid ref-counting the OwnedString which
should also increase performance.
This way, we will later be able to store additional information about
the node inside the same arena with a guarantee that they will always be
alive as long as the node is alive.
These additional information will include
a) the token's text (which can be a StringRef into a copy of the source
code that lives inside the SyntaxArena)
b) the token's unparsed trivia, which can be decomposed into pieces when
needed.
We cannot use unowned strings for token texts of incrementally parsed
syntax trees since the source buffer to which reused nodes refer will
have been freed for reused nodes. Always copying the token text whenever
OwnedString is passed is too expensive. A reference counted copy of the
string allows us to keep the token's string alive across incremental
parses while eliminating unnecessary copies.
