[clang][DependencyScanning] Preserve Necessary Preprocessor Callbacks during By-name Lookup (#197731)

The by-name lookup logic uses new dependency collector callbacks per
lookup. The algorithm used to wipe out all callbacks for each query.
This turned out to be perilous. We have two raw pointers in the
preprocessor that point to the callbacks, and removing all callbacks per
query can lead to use-after-free situations through these dangling
pointers. Resetting the dangling pointers to null does not really work
either, since there may be dependencies between the callbacks and other
data structures. An example of this is the `PreprocessingRecord *Record`
callback and the `GlobalPreprocessedEntityMap` in ASTReader. Hence, to
fix the use-after-free issue, we preserve the callbacks that the
preprocessor may hold a raw pointer to.

This is not intended to indicate how we want to handle this in the long
run. We should avoid removing PP callbacks and reset their states across
by-name lookups.

rdar://175362366