[libSyntax] Make RawSyntaxCacheNode retain its underlying RawSyntax node
[libSyntax] Make RawSyntaxCacheNode retain its underlying RawSyntax node

Previously RawSyntaxCacheNode was not increasing the RawSyntax refCount
and accessing a cached node would fail if the node had been deleted in
the meantime. We weren't hitting this so far, because all nodes were
allocated using a bump allocator and thus the underlying memory never
freed.

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