gdb: fix warnings about invalid [[fallthrough]] usage

Fix these two warnings, when building on macos:

      CXX    cp-name-parser.o
    /Users/smarchi/src/binutils-gdb/gdb/cp-name-parser.y:1644:7: error: fallthrough annotation does not directly precede switch label
          [[fallthrough]];
          ^

      CXX    dbxread.o
    /Users/smarchi/src/binutils-gdb/gdb/dbxread.c:2809:7: error: fallthrough annotation does not directly precede switch label
          [[fallthrough]];
          ^

In these two cases, we [[fallthrough]], followed by a regular label,
followed by a case label.  Move the [[fallthrough]] below the regular
label.

Change-Id: If4a3145139e050bdb6950c7f239badd5778e6f64
Approved-By: Tom Tromey <tom@tromey.com>