[sanitizer_common] Move filesystem-related code out of sanitizer_common.cc

This is a pure refactoring change. It just moves code that is
related to filesystem operations from sanitizer_common.{cc,h} to
sanitizer_file.{cc,h}. This makes it cleaner to disable the
filesystem-related code for a new port that doesn't want it.

Commiting for mcgrathr.

Reviewers: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35591

llvm-svn: 308640