Up until 5.2, ALWAYS, CLOSE, and PRESENT were keywords of the
map-type-modifier. Starting from 6.0 they all became their own
single-keyword modifiers. This allowed specifying them together,
unlike in the past where map-type-modifier was unique.
To avoid using a single representation of the modifiers, and be
able to validate them through non-conditional properties, the
AST was rewritten back to the older form in canonicalization
when the spec version was set to 5.2 or earlier.
Now that the parser is version-aware, it can generate the desired
AST from the start.
Additionally, extract the OMPX_HOLD modifier out of the map-type-
modifier into its own AST node regardless of version.
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