[PATCH] Add ability for can_write to indicate why the file is not writable

[Richard Smith]

Hi dblaikie,

Add an optional output parameter to `can_write` to allow it to indicate why a file is not considered writable (along with a sample use from Clang where we currently produce stupid diagnostics saying `"can't open output file: Success"`).

I'm not entirely sure this is the right design, and in particular it's a somewhat different interface from the other functions in this file. I also considered:

  std::error_code can_write(const Twine &Path, bool &result);

... which seems redundant because `result` is set to `true` iff the error code is `success`, and ...

  std::error_code can_write(const Twine &Path);

... which is terrible because `bool(can_write(Path))` is `true` iff the file is *not* writable, and ...

  std::error_code cannot_write(const Twine &Path);

... which is inconsistent with other functions near here and ugly.

http://reviews.llvm.org/D5182

Files:
  include/llvm/Support/FileSystem.h
  lib/Frontend/CompilerInstance.cpp
  lib/Support/Unix/Path.inc
  lib/Support/Windows/Path.inc
  test/Frontend/output-failures.c
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