[clang][dataflow] Add `create<T>()` methods to `Environment` and `DataflowAnalysisCon...

[clang][dataflow] Add `create<T>()` methods to `Environment` and `DataflowAnalysisContext`.

These methods provide a less verbose way of allocating `StorageLocation`s and
`Value`s than the existing `takeOwnership(make_unique(...))` pattern.

In addition, because allocation of `StorageLocation`s and `Value`s now happens
within the `DataflowAnalysisContext`, the `create<T>()` open up the possibility
of using `BumpPtrAllocator` to allocate these objects if it turns out this
helps performance.

Reviewed By: ymandel, xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D147302