BUG-7025: make the parser tree immutable

BUG-7025: make the parser tree immutable

This adds a post-processing phase, which walks the entire tree
and makes sure children are initialized to an ImmutableList.

This is essentially the same operation as running the tree
through org.antlr.v4.runtime.Parser.TrimToSizeListener, except
it also initializes empty lists to immutable collection and
uses a more efficient implementation for singletons.

Change-Id: I2cf484654158ce01bb9f7c2ac51a9866f88ae320
Signed-off-by: Robert Varga <rovarga@cisco.com>