[clang] [analyzer][NFC] Clean up `MemRegion.cpp` and align with llvm coding style (PR #218860)

[Shengxin Pei via cfe-commits]

TPPPP72 wrote:

@steakhal I am not sure exactly which single-line brace changes need to be reverted; judging by the context, the remaining modified blocks do not retain the braces, so I have not reverted the changes to them for the time being.

https://github.com/llvm/llvm-project/pull/218860