In the precense of multiple core libraries, `getAThrownException()` would return
multiple copies of the same exception, say `System.OverflowException`, one for each
core library. With this change we try to identify which core library a given control
flow element was compiled against, and only return the corresponding version.
Write accesses in assignments, such as the access to `x` in `x = 0` are not
evaluated, so they should not have entries in the control flow graph. However,
qualifiers (and indexer arguments) should still be evaluated, for example in
```
x.Foo.Bar = 0;
```
the CFG should be `x --> x.Foo --> 0 --> x.Foo.Bar = 0` (as opposed to
`x --> x.Foo --> x.Foo.Bar --> 0 --> x.Foo.Bar = 0`, prior to this change).
A special case is assignments via acessors (properties, indexers, and event
adders), where we do want to include the access in the control flow graph,
as it represents the accessor call:
```
x.Prop = 0;
```
But instead of `x --> x.set_Prop --> 0 --> x.Prop = 0` the CFG should be
`x --> 0 --> x.set_Prop --> x.Prop = 0`, as the setter is called *after* the
assigned value has been evaluated.
An even more special case is tuple assignments via accessors:
```
(x.Prop1, y.Prop2) = (0, 1);
```
Here the CFG should be
`x --> y --> 0 --> 1 --> x.set_Prop1 --> y.set_Prop2 --> (x.Prop1, y.Prop2) = (0, 1)`.
Using the class `ExceptionClass` in combination with a deliberate Cartesian
product can lead to bad join orders, for example
```
EVALUATE NONRECURSIVE RELATION:
Completion::TriedControlFlowElement::getAThrownException_dispred#ff(int this, int result) :-
{1} r1 = JOIN Expr::Expr::getType_dispred#ff_10#join_rhs WITH @integral_type#f ON Expr::Expr::getType_dispred#ff_10#join_rhs.<0>=@integral_type#f.<0> OUTPUT FIELDS {Expr::Expr::getType_dispred#ff_10#join_rhs.<1>}
{1} r2 = JOIN r1 WITH @un_op#f ON r1.<0>=@un_op#f.<0> OUTPUT FIELDS {r1.<0>}
{1} r3 = JOIN r2 WITH Stmt::TryStmt::getATriedElement#ff_1#join_rhs ON r2.<0>=Stmt::TryStmt::getATriedElement#ff_1#join_rhs.<0> OUTPUT FIELDS {r2.<0>}
{2} r4 = JOIN r3 WITH Stmt::ExceptionClass#f CARTESIAN PRODUCT OUTPUT FIELDS {Stmt::ExceptionClass#f.<0>,r3.<0>}
{2} r5 = JOIN r4 WITH System::SystemOverflowExceptionClass#class#f ON r4.<0>=System::SystemOverflowExceptionClass#class#f.<0> OUTPUT FIELDS {r4.<1>,r4.<0>}
```
where the CP is made with `ExceptionClass` rather than `SystemOverflowExceptionClass`
directly.
