[sanitizer] Check if directory exists before trying to create

Add a DirExists mechanism, modeled after FileExists. Use it to guard
creation of the report path directory.

This should avoid failures running the sanitizer in a sandbox where the
file creation attempt causes hard failures, even for an existing
directory. Problem reported on D109794 for ChromeOS in sandbox
(https://issuetracker.google.com/209296420).

Differential Revision: https://reviews.llvm.org/D119495