Remove implicit fallthrough (#3518)

This is intended so that downstream projects consuming glslang with FetchContent or similar means can use `-Wimplicit-fallthrough` without getting warning spam.

I've used my best judgement to determine whether the implicit fallthrough was desired, or was simply unreachable code.
`std::unreachable` is unavailable until C++23, but I saw places where `default: assert(0);` was used, so copied that.
There were a few places where some code might actually have been reachable and intended to return a value that represented an error, so someone should double check that kind of thing.

SPIRV/SpvPostProcess.cpp

@@ -181,6 +181,7 @@ void Builder::postProcessType(const Instruction& inst, Id typeId)
             else if (width == 8)
                 addCapability(CapabilityInt8);
         }
+        break;
     default:
         if (basicTypeOp == OpTypeInt) {
             if (width == 16)