Code assist within guard predicates

[stephan]

Code assist for the expression of a guard predicate should be implemented, specifically: select & complete.

The basic issue here is syntax recovery within guard predicates (thus assigning to "compiler").

[stephan]

Selection for base.m() within base guard is fixed in r19361:

• double parsing of predicate led to interpeting this expression as a base-call.

[stephan]

r19362 and r19363 change how the parser consumes guard predicates:

• need to create predicate method before parsing the expression so that the completion parser will find the predicate even if parsing stops before the end of the expression.
• include predicates in the class header so the completion parser will find them in recovery mode, too.

[stephan]

r19364 finishes syntax recovery for type-level predicates and adds a few tests: PASS.