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.

glslang/MachineIndependent/Intermediate.cpp

@@ -1277,6 +1277,7 @@ void TIntermediate::addBiShapeConversion(TOperator op, TIntermTyped*& lhsNode, T
         // matrix multiply does not change shapes
         if (lhsNode->isMatrix() && rhsNode->isMatrix())
             return;
+        [[fallthrough]];
     case EOpAdd:
     case EOpSub:
     case EOpDiv: