[clang][dataflow] Replace initValueInStorageLocation with createValue

Since Environment's setValue method already does part of the work that
initValueInStorageLocation does, we can factor out a new createValue
method to reduce the duplication.

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D117493