[clang][dataflow][NFC] Eliminate StmtToEnvMap interface.

Instead, we turn StmtToEnvMap into a concrete class with the implementation that used to live in StmtToEnvMapImpl.

The layering issue that originally required the indirection through the
`StmtToEnvMap` interface no longer exists.

Reviewed By: ymandel, xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D146507