Add internal API to fold FlowAnalysisAnnotations into the symbol's TypeWithAnnotations

[RikkiGibson]

Basically if you just access something like the TypeWithAnnotations on a symbol, and you don't try to "fold in" the FlowAnalysisAnnotations on that symbol into the TypeWithAnnotations, you probably have bugs related to nullability attributes. It does feel like there should be something like GetAdjustedTypeWithAnnotations() (strawman name) on certain kinds of symbols in order to make it easier to do the right thing.

Originally posted by @RikkiGibson in #47555 (comment)

[RikkiGibson]

Closing as stale.