Initialize symbols for primitive types on compiler startup.

The nightly build uncovered a race condition in the indexer, that stepped on
a case where `definitions.ShortClass` was not yet loaded, triggering the
symbol loader.

A race condition may happen inside `mapType`, where a symbol is pattern-matched against every possible primitive type. If those primitive types (like `definition.UnitClass`, etc.) have not yet been loaded, the compiler will load them (the usual laziness). To protect against that, one needs an `ask` call, but since these primitive types should be loaded once, and no other part of the method touches symbols, we optimize by making sure they are loaded always, and skip the `ask` call.

Needs back port. Should fix flaky presentation compiler tests.