Keep multiple-include optimization for null directives

The multiple-include optimization allows Clang to avoid opening a
files when they contain #pragma once or a proper include guard.

Both GCC and Microsoft Visual Studio allow null directives outside of
the #ifndef/#endif pair without disabling this multiple-include
optimization. GCC documents this behavior here
https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html.

> There must be no directives outside the controlling directive pair,
> but the null directive (a line containing nothing other than a
> single '#' and possibly whitespace) is permitted.

However, Clang disables the multiple-include optimization when
encountering the null directive.

In particular, this slows down preprocessing of most projects that
depend on boost as many boost libraries depend on the boost
preprocessor library, which contains null directives outside the
include guard on every header file.

Differential Revision: https://reviews.llvm.org/D147928