Pass a FD to resise_file and add a testcase.

I will add a real use in another commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224136 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/FileSystem.h

@@ -336,11 +336,11 @@ std::error_code copy_file(const Twine &From, const Twine &To);
 
 /// @brief Resize path to size. File is resized as if by POSIX truncate().
 ///
-/// @param path Input path.
+/// @param path Input file descriptor.
 /// @param size Size to resize to.
 /// @returns errc::success if \a path has been resized to \a size, otherwise a
 ///          platform-specific error_code.
-std::error_code resize_file(const Twine &path, uint64_t size);
+std::error_code resize_file(int FD, uint64_t Size);
 
 /// @}
 /// @name Physical Observers