[clang][dataflow] Use `Strict` accessors in more places in Transfer.cpp.

This patch handles the straightforward cases. Upcoming separate patches will handle the cases that are more subtle.

This patch is part of the ongoing migration to strict handling of value categories (see https://discourse.llvm.org/t/70086 for details).

Depends On D150653

Reviewed By: sammccall, ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D150655