[Umbrella] Switch stmt CIRGen issues

[bcardosolopes]

There are many current issues with switch statements, mostly regarding all weird supported combinations. This issue is aggregating all of them in order to facilitate folks to coordinate and execute.

• Assertion failure on switch statement with code outside a case: https://github.com/llvm/clangir/issues/521
• Assertion failure on switch statement with non-block substatement: https://github.com/llvm/clangir/issues/520
• Assertion failure on switch statement with case label in nested block: https://github.com/llvm/clangir/issues/522
• Miss destruction in switch statements: https://github.com/llvm/clangir/issues/587
• Module verification error of switch statements: https://github.com/llvm/clangir/issues/588
• Incorrect terminator generation: https://github.com/llvm/clangir/issues/602

CC: @wenpen @piggynl @gitoleg