tsan: output stats about stackdepot in memory profiler

quite amusing on some programs:
RSS 9152 MB: shadow:2456 meta:1736 file:454 mmap:3442 trace:196 heap:866 other:0 stacks=176[1065994] nthr=157/159
...
RSS 30192 MB: shadow:613 meta:5550 file:1839 mmap:20487 trace:212 heap:1489 other:0 stacks=12870[1599445] nthr=162/165
...
RSS 42232 MB: shadow:599 meta:8199 file:2304 mmap:29420 trace:213 heap:1494 other:0 stacks=17794[1690645] nthr=162/165

llvm-svn: 218988