[PATCH] D50539: [VFS] Add property 'fallthrough' that controls fallback to real file system.

[Volodymyr Sapsai via Phabricator via cfe-commits]

vsapsai created this revision.
vsapsai added reviewers: bruno, benlangmuir.
Herald added a subscriber: dexonsmith.

Default property value 'true' preserves current behavior. Value 'false' can be
used to create VFS "root", file system that gives better control over which
files compiler can use during compilation as there are no unpredictable
accesses to real file system.

Non-fallthrough use case changes how we treat multiple VFS overlay
files. Instead of all of them being at the same level just above a real
file system, now they are nested and subsequent overlays can refer to
files in previous overlays.

rdar://problem/39465552


https://reviews.llvm.org/D50539

Files:
  clang/lib/Basic/VirtualFileSystem.cpp
  clang/lib/Frontend/CompilerInvocation.cpp
  clang/test/VFS/Inputs/Broken.framework/Headers/Error.h
  clang/test/VFS/Inputs/Broken.framework/Modules/module.modulemap
  clang/test/VFS/Inputs/Broken.framework/VFSHeaders/A.h
  clang/test/VFS/Inputs/vfsroot.yaml
  clang/test/VFS/vfsroot-include.c
  clang/test/VFS/vfsroot-module.m
  clang/test/VFS/vfsroot-with-overlay.c
  clang/unittests/Basic/VirtualFileSystemTest.cpp

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