[clang-scan-deps] reuse the file manager across invocations of

[clang-scan-deps] reuse the file manager across invocations of
the dependency scanner on a single worker thread

This behavior can be controlled using the new `-reuse-filemanager` clang-scan-deps
option. By default the file manager is reused.

The added test/ClangScanDeps/symlink.cpp is able to pass with
the reused filemanager after the related FileEntryRef changes
landed earlier. The test test/ClangScanDeps/subframework_header_dir_symlink.m
still fails when the file manager is reused (I run the FileCheck with not to
make it PASS). I will address this in a follow-up patch that improves
the DirectoryEntry name modelling in the FileManager.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370420 91177308-0d34-0410-b5e6-96231b3b80d8