Add descriptive names to sanitizer entries in /proc/self/maps. Helps debugging.

This is done by creating a named shared memory region, unlinking it
and setting up a private (i.e. copy-on-write) mapping of that instead
of a regular anonymous mapping. I've experimented with regular
(sparse) files, but they can not be scaled to the size of MSan shadow
mapping, at least on Linux/X86_64 and ext3 fs.

Controlled by a common flag, decorate_proc_maps, disabled by default.

This patch has a few shortcomings:
* not all mappings are annotated, especially in TSan.
* our handling of memset() of shadow via mmap() puts small anonymous
  mappings inside larger named mappings, which looks ugly and can, in
  theory, hit the mapping number limit.

llvm-svn: 238621