This commit adds field initializers to the CFG for non-static constructors. For
example, in
```
class C
{
int Field1 = 0;
int Field2 = Field1 + 1;
int Field3;
public C()
{
Field3 = 2;
}
public C(int i)
{
Field3 = 3;
}
}
```
the initializer expressions `Field1 = 0` and `Field2 = Field1 + 1` are added
to the two constructors, mimicking
```
public C()
{
Field1 = 0;
Field2 = Field1 + 1;
Field3 = 2;
}
```
and
```
public C()
{
Field1 = 0;
Field2 = Field1 + 1;
Field3 = 3;
}
```
respectively. This means that we no longer have to synthesize calls, callables,
parameters, and arguments in the data flow library, so much of the work from
d1755500e4 can be simplified.
Initial implementation of data flow through fields, using the algorithm of the
shared data flow implementation. Fields (and field-like properties) are covered,
and stores can be either
- ordinary assignments, `Foo = x`,
- object initializers, `new C() { Foo = x }`, or
- field initializers, `int Foo = x`.
For field initializers, we need to synthesize calls (`SynthesizedCall`),
callables (`SynthesizedCallable`), parameters (`InstanceParameterNode`), and
arguments (`SynthesizedThisArgumentNode`), as the C# extractor does not (yet)
extract such entities. For example, in
```
class C
{
int Field1 = 1;
int Field2 = 2;
C() { }
}
```
there is a synthesized call from the constructor `C`, with a synthesized `this`
argument, and the targets of that call are two synthesized callables with bodies
`this.Field1 = 1` and `this.Field2 = 2`, respectively.
A consequence of this is that `DataFlowCallable` is no longer an alias for
`DotNet::Callable`, but instead an IPA type.
- Extract names of properties in a propery match, using the `exprorstmt_name` relation.
- Simplify extraction of properties by not distinguishing between top-level patterns
and nested patterns.
- Introduce `PatternExpr` to capture patterns in `is` expressions, `case` statements,
and `switch` expression arms.
- Generalize `IsTypeExpr`, `IsPatternExpr`, `IsRecursivePatternExpr`, and `IsConstantExpr`
to just `IsExpr` with a member predicate `PatternExpr getPattern()`.
- Generalize `TypeCase`, `RecursivePatternCase`, and `ConstCase` to just `CaseStmt` with
a member predicate `PatternExpr getPattern()`.
- Introduce classes `Switch` and `Case` as base classes of switch statements/expressions
and case statements/switch expression arms, respectively.
- Simplify CFG logic using the generalized classes.
- Generalize guards library to cover `switch` expressions tests.
- Generalize data flow library to cover `switch` expression assignments.
