[clang][dataflow][NFC] Share code between Environment ctor and pushCallInternal().

The deduplicated code is moved into initVars().

As an added bonus, pushCallInternal() now also gets the "Add all fields
mentioned in default member initializers" behavior, which apparently had been
added to the Environment ctor but not pushCallInternal().

Reviewed By: xazax.hun, ymandel

Differential Revision: https://reviews.llvm.org/D147326