Fix a bug that made the nightly tester *really* slow. During changes for

portability, the --track-space option was inadvertently ignored. This patch
fixes that so that sys::Process::GetMallocUsage() is only invoked if the
--track-spaces option is given. Apparently the mallinfo() call that
GetMallocUsage() uses is *very* slow, especially when processing very large
modules like projects/llvm-test/MultiSource/Applications/kimwitu++.

llvm-svn: 19163