[clang][dataflow] Use `Strict` accessors in SignAnalysisTest.cpp.

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 D150656

Reviewed By: sammccall, ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D150657