...in order to stop memory intensive parsing for invalid code.

Parsing the test data/snippet "hard" led to a memory consumption of
about 5.5MB and this could easily get up to hundreds/gigabytes by adding
some more "if_<bool_<true>,\n" lines. With the caching, we are at about
1.0MB, even if more lines are added.

The "memory consumption" was measured with valgrind-massif. The stated
numbers are the reported peaks.

Task-number: QTCREATORBUG-12890
Change-Id: Ie7eb00cfc7915552d29bb27410a6b13a486f486e
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>