#247 Implement pragma once directive.

[TIMONz1535]

• When the lexer parses pragma directive with the first keyword once, it adds current File.FilePath to the HashSet.
• Then when the lexer parses include directive it checks if it Contains include.FilePath and doesn't invoke PushIncludeContext (doesn't include the file content).
• Added test for #pragma once but without path aliases, because InMemoryFileSystem can't handle it properly.
• Regarding to DXC bug #pragma once cannot support path aliases microsoft/DirectXShaderCompiler#3943 it also cannot support path aliases. It is also case sensitive (not a bug really, its Windows magic). Well its correlates with the latest DXC release!

objects:
	header.hlsli:
		#pragma once
	object.hlsl:
		#include "header.hlsli" // valid
		#include "header.hlsli" // valid, skip

shading.hlsl:
	#include "objects/header.hlsli" // valid, skip
	#include "objects/object.hlsl" // skip header, but not object
	#include "objects/Header.hlsli" // redefinition, because don't do that
	// dxc bug
	#include "fake/../objects/header.hlsli" // on no, redefinition
	#include "fake/../objects/object.hlsl" // on no, another redefinition of header

If you don't know how to resolve the conflict with VariadicArgument, just put OnceKeyword after WarningKeyword but before VariadicArgument.