[SyntaxTree][Synthesis] Fix allocation in `createTree` for more general use

Prior to this change `createTree` could not create arbitrary syntax
trees. Now it dispatches to the constructor of the concrete syntax tree
according to the `NodeKind` passed as argument. This allows reuse inside
the Synthesis API.  # Please enter the commit message for your changes.
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Differential Revision: https://reviews.llvm.org/D87820