[clang][lex] Keep references to `DirectoryLookup` objects up-to-date

The elements of `SearchPath::SearchDirs` are being referenced to by their indices. This proved to be error-prone: `HeaderSearch::SearchDirToHSEntry` was accidentally not being updated in `HeaderSearch::AddSearchPath()`. This patch fixes that by referencing `SearchPath::SearchDirs` elements by their address instead, which is stable thanks to the bump-ptr-allocation strategy.

Reviewed By: ahoppen

Differential Revision: https://reviews.llvm.org/D116750