Breakpoint conditions were making result variables, which they should not do.

The result variables aren't useful, and if you have a breakpoint on a
common function you can generate a lot of these.  So I changed the
code that checks the condition to set ResultVariableIsInternal in the
EvaluateExpressionOptions that we pass to the execution.
Unfortunately, the check for this variable was done in the wrong place
(the static UserExpression::Evaluate) which is not how breakpoint
conditions execute expressions (UserExpression::Execute).  So I moved
the check to UserExpression::Execute (which Evaluate also calls) and made the
overridden method DoExecute.

llvm-svn: 266093