Line directive has optional filename argument.

[TIMONz1535]

• Filename string argument in the line directive is optional https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-appendix-pre-line
• If this happens then in ParseLineDirective I create the missing token EndOfDirectiveToken. I'm not sure if its ok, but it works.
• HlslLexer.LexDirectiveToken must reset his value _value = null; before parsing, like HlslLexer.LexSyntaxToken. Because some directives have no value and it extrapolates the previous one to next - bad.
• Removing afterLineNumber argument from ParseEndOfDirective. It looks like a special error for the line directive, it referred to MissingPreprocessorFile but was never used.
• Added a test for #line without filename, it also checking if the end of directive is parsed correctly.
• Fix test checking for LineDirectiveTrivia kind. No more special cases for 0, -1, -2 line number. It just should be >= 0. Yes, correlates with the latest DXC release!

#line 10 "file.hlsl"  // valid
#line 20              // valid too