[Coverage] Emit a gap region to cover switch bodies

Emit a gap region beginning where the switch body begins. This sets line
execution counts in the areas between non-overlapping cases to 0.

This also removes some special handling of the first case in a switch:
these are now treated like any other case.

This does not resolve an outstanding issue with case statement regions
that do not end when a region is terminated. But it should address
llvm.org/PR44011.

Differential Revision: https://reviews.llvm.org/D70571