Minor improvements to the simplified expression flags

Two related changes:

a) It would be clearer if ignoreCache was called ignoreSimplified - since it is also use to suppress whether or not a simplified expression that does not come from the cache is used.

b) The code in parseAttribute should only create a simplified definition if it is actually going to be used.  I think that means the test

    if (canCache && (ctx.syntaxChecking() || ctx.hasCacheLocation()))

should become

    if (canCache && (ctx.syntaxChecking() && (ctx.checkSimpleDef || !ctx.ignoreCache)) || ctx.hasCacheLocation()

possibly with a new ctx method ctx.createSimplified()