Support: Don't set RLIMIT_AS on child processes when applying a memory limit

It doesn't seem relevant to set an address space limit - this isn't
important in any sense that I'm aware & it gets in the way of things
that use a lot of address space, like llvm-symbolizer.

This came up when I realized that bugpoint regression tests were much
slower with -gsplit-dwarf than plain -g. Turned out that bugpoint
subprocesses (opt, etc) were crashing and doing symbolization - but
bugpoint runs those subprocesses with a 400MB memory limit. So with
plain -g, mmaping the opt binary would exceed the memory limit, fail,
and thus be really fast - no symbolization occurred. Whereas with
-gsplit-dwarf, comically, having less to map in, it would succeed and
then spend lots of time symbolizing.

I've fixed at least the critical part of bugpoint's perf problem there
by adding an option to allow bugpoint to disable symbolization. Thus
improving the perfromance for -gsplit-dwarf and making the -g-esque
speed available without this quirk/accidental benefit.

llvm-svn: 305242