[pseudo] GC GSS nodes, reuse them with a freelist

Most GSS nodes have short effective lifetimes, keeping them around until the
end of the parse is wasteful. Mark and sweep them every 20 tokens.

When parsing clangd/AST.cpp, this reduces the GSS memory from 1MB to 20kB.
We pay ~5% performance for this according to the glrParse benchmark.
(Parsing more tokens between GCs doesn't seem to improve this further).

Compared to the refcounting approach in https://reviews.llvm.org/D126337, this
is simpler (at least the complexity is better isolated) and has >2x less
overhead. It doesn't provide death handlers (for error-handling) but we have
an alternative solution in mind.

Differential Revision: https://reviews.llvm.org/D126723