[clang][dataflow] Handle multiple context-sensitive calls to the same function

This patch enables context-sensitive analysis of multiple different calls to the same function (see the `ContextSensitiveSetBothTrueAndFalse` example in the `TransferTest` suite) by replacing the `Environment` copy-assignment with a call to the new `popCall` method, which  `std::move`s some fields but specifically does not move `DeclToLoc` and `ExprToLoc` from the callee back to the caller.

To enable this, the `StorageLocation` for a given parameter needs to be stable across different calls to the same function, so this patch also improves the modeling of parameter initialization, using `ReferenceValue` when necessary (for arguments passed by reference).

This approach explicitly does not work for recursive calls, because we currently only plan to use this context-sensitive machinery to support specialized analysis models we write, not analysis of arbitrary callees.

Reviewed By: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D130726