David Farler c343298b8f [Syntax] Implement return-statement and integer-literal-expr ...

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

2017-02-22 18:45:29 -08:00

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