[GNU/ELF] Add support for -X/--discard-locals.

There's (almost) never need to keep .L symbols around for production
builds. In fact, the FreeBSD kernel explicitly specify -X beacuse the
size impact (and the subsequent performance impact) might be significant,
because we keep symbols in memory.
I was tempted to make this the default, but I haven't (yet).

PR:		23232
llvm-svn: 235357