This will make it easier to incrementally implement syntax nodes,
while allowing us to embed nodes that we do know about inside ones
that we don't.
https://bugs.swift.org/browse/SR-4062
Also includes for its substructure:
- function-call-argument
- function-call-argument-list
- symbolic-reference-expression (for the call target)
https://bugs.swift.org/browse/SR-4044
A return statement needs something to return, so implement
integer-literal-expression too. This necessarily also forced
UnknownExprSyntax, UnknownStmtSyntax, and UnknownDeclSyntax,
which are stand-in token buckets for when we don't know
how to transform/migrate an AST.
This commit also contains the core function for caching
SyntaxData children. This is highly tricky code, with some
detailed comments in SyntaxData.{h,cpp}. The gist is that
we have to atomically swap in a SyntaxData pointer into the
child field, so we can maintain pointer identity of SyntaxData
nodes, while still being able to cache them internally.
To prove that this works, there is a multithreaded test that
checks that two threads can ask for a child that hasn't been
cached yet without crashing or violating pointer identity.
https://bugs.swift.org/browse/SR-4010
