[Windows] Fix assertion failure when passing 'nul' in input to clang.

Before this patch, if 'nul' was passed in input to clang, function
getStatus() (in Path.inc) always returned an instance of file_status with
field 'nFileSizeHigh' and 'nFileSizeLow' left uninitialized.

This was causing the triggering of an assertion failure in MemoryBuffer.cpp due
to an invalid FileSize for device 'nul'.

This patch fixes the assertion failure modifying the constructors of class
file_status (in llvm/Support/FileSystem.h) so that every field of the class
gets initialized to zero by default.

A clang test will be submitted on a separate patch.