Change `NilLiteralExpr_create` to `NilLiteralExpr.init` etc.

[ahoppen]

The NilLiteralExpr_create-style methods look a little foreign in Swift and ideally, they would be constructor calls. This issue talks about the NilLiteralExpr_create function, but it really applies to all the _create functions.

Depending on what we decide to do in #68346, we have three options:

• Use C++ interop and make the create method static members on NilLiteralExpr
• Use C++ interop to introduce the NilLiteralExpr type in Swift and import the create method as a constructor on NilLiteralExpr using the SWIFT_NAME attribute (at least I think this should be possible)
• If we choose to introduce BridgedNilLiteralExpr, we should import NilLiteralExpr_create as an initializer on BridgedNilLiteralExpr using the SWIFT_NAME attribute

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Similarly, the AbstractFunctionBody_setBody-style functions should be imported as member functions on the underlying type.