`core::SelectOp` and some `void` related fixes.

[lkorenc]

Adds lowering of core::SelectOp. Fixes some corner cases related to void "values" being returned.

[github-actions[bot]]

Cpp-Linter Report :warning:

Some files did not pass the configured checks!

clang-format reports: 6 file(s) not formatted

• include/vast/Conversion/Common/Patterns.hpp
• include/vast/Conversion/TypeConverters/LLVMTypeConverter.hpp
• lib/vast/Conversion/Core/ToLLVM.cpp
• lib/vast/Conversion/FromHL/ToLLCF.cpp
• lib/vast/Conversion/ToLLVM/Common.hpp
• lib/vast/Conversion/ToLLVM/IRsToLLVM.cpp

clang-tidy reports: 2 concern(s)

• include/vast/Conversion/TypeConverters/LLVMTypeConverter.hpp:5:1: error: [clang-diagnostic-error]

  unknown type name 'VAST_RELAX_WARNINGS'

      5 | VAST_RELAX_WARNINGS
        | ^
  

• include/vast/Conversion/TypeConverters/LLVMTypeConverter.hpp:15:1: error: [clang-diagnostic-error]

  unknown type name 'VAST_UNRELAX_WARNINGS'

     15 | VAST_UNRELAX_WARNINGS
        | ^
  

Have any feedback or feature suggestions? Share it here.