[clang][AST] fix ast-print of `extern <lang>` with >=2 declarators (#93131)

Problem: the printer used to ignore all but the first declarator for
unbraced language linkage declarators. Furthemore, that one would be
printed without the final semicolon.

Solution: when there is more than one declarator, we print them in a
braced `extern <lang>` block. If the original declaration was unbraced
and there is one or less declarator, we omit the braces, but add the
semicolon.

**N.B.** We are printing braces which were, in some cases, absent from
the original CST. If that's an issue, I'll work on it. See the tests for
the examples.