Apply proper source location to fallthrough switch cases.

This fixes a bug in clang where, when clang sees a switch with a
fallthrough to a default like this:

static void funcA(void) {}
static void funcB(void) {}

int main(int argc, char **argv) {

switch (argc) {
    case 0:
        funcA();
        break;
    case 10:
    default:
        funcB();
        break;
}
}

It does not add a proper debug location for that switch case, such as
case 10: above.

Patch by Shubham Rastogi!

Differential Revision: https://reviews.llvm.org/D109940