[Lex/DependencyDirectivesScanner] Keep track of the presence of tokens between...

[Lex/DependencyDirectivesScanner] Keep track of the presence of tokens between the last scanned directive and EOF

Directive `dependency_directives_scan::tokens_present_before_eof` is introduced to indicate there were tokens present before
the last scanned dependency directive and EOF.
This is useful to ensure we correctly identify the macro guards when lexing using the dependency directives.

Differential Revision: https://reviews.llvm.org/D133357