[clang][lex] NFC: Simplify calls to `LookupFile`

The `{HeaderSearch,Preprocessor}::LookupFile()` functions take an out-parameter `const DirectoryLookup *&`. Most callers end up creating a `const DirectoryLookup *` variable that's otherwise unused.

This patch changes the out-parameter from reference to a pointer, making it possible to simply pass `nullptr` to the function without the ceremony.

Reviewed By: ahoppen

Differential Revision: https://reviews.llvm.org/D117312