Revert "[-Wunsafe-buffer-usage] Group variables associated by pointer assignments"

This reverts commit ee6b08e9.

One of the added tests warn-unsafe-buffer-usage-multi-decl-warnings.cpp does
not seem to be deterministic, and seems to be especially problematic on Windows.

Failures of this one test on llvm-clang-x86_64-sie-win:
- https://lab.llvm.org/buildbot/#/builders/216/builds/21758
- https://lab.llvm.org/buildbot/#/builders/216/builds/21761
- https://lab.llvm.org/buildbot/#/builders/216/builds/21762
- https://lab.llvm.org/buildbot/#/builders/216/builds/21765
- https://lab.llvm.org/buildbot/#/builders/216/builds/21770
- https://lab.llvm.org/buildbot/#/builders/216/builds/21771
- https://lab.llvm.org/buildbot/#/builders/216/builds/21773
- https://lab.llvm.org/buildbot/#/builders/216/builds/21776
- https://lab.llvm.org/buildbot/#/builders/216/builds/21777
- https://lab.llvm.org/buildbot/#/builders/216/builds/21778
- https://lab.llvm.org/buildbot/#/builders/216/builds/21779

Other random bot failures:
- https://lab.llvm.org/buildbot/#/builders/65/builds/9821
- https://lab.llvm.org/buildbot/#/builders/65/builds/9822
- https://lab.llvm.org/buildbot/#/builders/65/builds/9824
- https://lab.llvm.org/buildbot/#/builders/119/builds/13440
- https://lab.llvm.org/buildbot/#/builders/119/builds/13442
- https://lab.llvm.org/buildbot/#/builders/119/builds/13444
- https://lab.llvm.org/buildbot/#/builders/119/builds/13445
- https://lab.llvm.org/buildbot/#/builders/60/builds/12156
- https://lab.llvm.org/buildbot/#/builders/60/builds/12157
- https://lab.llvm.org/buildbot/#/builders/60/builds/12160