[clang/test/CodeGen] Add test coverage for VarBypassDetector handling init statements...

[clang/test/CodeGen] Add test coverage for VarBypassDetector handling init statements and condition variables in switch clauses

[VarBypassDetector.cpp](https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/VarBypassDetector.cpp) lacks test coverage for handling init statements and condition variables in switch clauses, as is shown in:
https://lab.llvm.org/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/clang/lib/CodeGen/VarBypassDetector.cpp.html#L70
This patch adds test coverage for uncovered lines.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D152412