Maintenance: Trigger -Wswitch in more build environments (#1104)

Maintenance: Trigger -Wswitch in more build environments (#1104)

When a developer forgets to update RawSwapMetaTypeTop() after adding a
new SwapMetaType item, the function should produce a -Wswitch compiler
warning (at least). However, we discovered that many compilers remain
silent, apparently confused by the constant switch expression:

* GCC warns only starting with v9.1;
* clang does not warn at all (up to v14, inclusive).

Adding a non-constant variable triggers the expected -Wswitch warnings
in clang and earlier GCC versions. Optimizing compilers (-O1) remove the
otherwise unused variable so adding it has no runtime cost.

Declaring the variable outside the switch statement is required to avoid
an unwanted -Wunused-variable warning with GCC v4.8 and clang v3.0.0.

A post-switch return statement is required to avoid an unwanted
-Wreturn-type warning with GCC v4.9.

Tests: https://gcc.godbolt.org/z/d1czs4c8e