[clang-tidy] NFC refactor lexer-utils to be usable without ASTContext

Summary:
This patch is a small refactoring necessary for
'readability-isolate-declaration' and does not introduce functional changes.
It allows to use the utility functions without a full `ASTContext` and requires only the `SourceManager` and the `LangOpts`.

Reviewers: alexfh, aaron.ballman, hokein

Reviewed By: alexfh

Subscribers: nemanjai, xazax.hun, kbarton, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D52684

llvm-svn: 343850