Not work for rustc_driver

[Jaic1]

The dhat::Profiler profiler gives zero heap stats (i.e., not allocating any blocks) when I use the rustc_driver crate to start a rustc instance. After debugging, I find that dhat::Alloc::alloc does not get called.

@nnethercote I wonder if this problem is relevant to the fact that rustc has a special mechanism to use jemalloc. BTW, valgrind --tool=dhat works for my project (already helped resolving OOM problems but took dozens of hours to run), and I guess dhat-rs would run significantly faster.