[clang][deps] Disable FileManager sharing with CASFS

[jansvoboda11]

The CAS-based scanner relies on each module accessing the VFS, which sharing of FileManager prevents. This fixes the following tests:

  Clang :: ClangScanDeps/modules-cas-fs-prefix-mapping-caching.c
  Clang :: ClangScanDeps/modules-cas-fs-prefix-mapping.c
  Clang :: ClangScanDeps/modules-cas-fs-symlink-dir-from-module.c
  Clang :: ClangScanDeps/modules-cas-full-by-mod-name.c
  Clang :: ClangScanDeps/modules-cas-trees.c
  Clang :: ClangScanDeps/modules-pch-cas-fs-prefix-mapping-caching.c
  Clang :: ClangScanDeps/modules-pch-cas-fs-prefix-mapping.c