[analyzer][NFCi] Annotate major nonnull returning functions

This patch annotates the most important analyzer function APIs.
Also adds a couple of assertions for uncovering any potential issues
earlier in the constructor; in those cases, the member functions were
already dereferencing the members unconditionally anyway.

Measurements showed no performance impact, nor crashes.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D126198