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

Summary:
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.

Submitted on behalf of Roland McGrath.

Reviewers: kcc, eugenis, alekseyshl

Reviewed By: alekseyshl

Subscribers: vitalybuka, llvm-commits, kubamracek, mgorny, phosek

Tags: #sanitizers

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

llvm-svn: 308819