Add support for IfThen() instrinsic that works like the if-statement in that it evaluates only the expression that is indeed executed.
The result type of the intrinsic is determined by the Then-expression to provide a bit of control. There might however be some situations in which this fails, for this exceptions need to be added (e.g. a constant string needs to be converted to a normal string).

compinnr.inc:
  + add new constant in_ifthen_x_y_z for the IfThen() intrinsic
psystem.pas: 
  + create_intern_symbols: add symbol for IfThen() intrinsic
pexpr.pas:
  * statement_syssym: parse parameters of IfThen() intrinsic and return corresponding inline node
ninl.pas, tinlinenode:
  + new method handle_ifthen() which converts the inline node to an if-node which assigns the expressions to a temp node that is returned
  * pass_typecheck: handle in_ifthen_x_y_z using handle_ifthen()
  * pass_1: in_ifthen_x_y_z does not need a first pass as it's already converted after the typecheck pass

+ added tests

git-svn-id: trunk@33036 -