[clang][lex] Remark on search path usage

For dependency scanning, it would be useful to collect header search paths (provided on command-line via `-I` and friends) that were actually used during preprocessing. This patch adds that feature to `HeaderSearch` along with a new remark that reports such paths as they get used.

Previous version of this patch tried to use the existing `LookupFileCache` to report used paths via `HitIdx`. That doesn't work for `ComputeUserEntryUsage` (which is intended to be called *after* preprocessing), because it indexes used search paths by the file name. This means the values get overwritten when the code contains `#include_next`.

Note that `HeaderSearch` doesn't use `HeaderSearchOptions::UserEntries` directly. Instead, `InitHeaderSearch` pre-processes them (adds platform-specific paths, removes duplicates, removes paths that don't exist) and creates `DirectoryLookup` instances. This means we need a mechanism for translating between those two. It's not possible to go from `DirectoryLookup` back to the original `HeaderSearch`, so `InitHeaderSearch` now tracks the relationships explicitly.

Depends on D111557.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D102923