[clang][dataflow] Add cache of `ControlFlowContext`s for function decls.

This patch modifies context-sensitive analysis of functions to use a cache,
rather than recreate the `ControlFlowContext` from a function decl on each
encounter. However, this is just step 1 (of N) in adding support for a
configurable map of "modeled" function decls (see issue #56879). The map will go
from the actual function decl to the `ControlFlowContext` used to model it. Only
functions pre-configured in the map will be modeled in a context-sensitive way.

We start with a cache because it introduces the desired map, while retaining the
current behavior. Here, functions are mapped to their actual implementations
(when available).

Differential Revision: https://reviews.llvm.org/D131039

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