Fix inconsistent treatment of generated code with restrictive annotations

[lazaroclapp]

Without this change, we have a bug whenever the following flags are
used together:

-XepOpt:NullAway:TreatGeneratedAsUnannotated=true
-XepOpt:NullAway:AcknowledgeRestrictiveAnnotations=true

and a method inside @Generated code is explicitly marked as having a
@Nullable return.

In this case, the RestrictiveAnnotationHandler would treat the method
as definitely @NonNull when handling the onOverrideMayBeNullExpr extension
point, but as @Nullable for the purposes of onDataflowVisitMethodInvocation.

This is inconsistent on its own, but it produces even more undesirable behavior
if a castToNonNull method is configured (whether through library models or
-XepOpt:NullAway:CastToNonNullMethod).

If foo is such a @Generated+@Nullable method, then, in the above
configuration:

Object o = castToNonNull(foo());
o.toString();

Produces an error, claiming that a known @NonNull value is being passed to
the cast.

Yet the following code:

Object o = foo();
o.toString();

Causes NullAway to complain about a @Nullable dereference (due to dataflow
concluding o is @Nullable).

To fix this, we need to make sure that the behavior of AcknowledgeRestrictiveAnnotations
is consistent.

One option is to not care about @Generated vs non-generated code, or
about TreatGeneratedAsUnannotated, and always use restrictive annotations
if available. Unfortunatelly, this is a change that will increase the number of
errors from NullAway on existing codebases (internally, we use
TreatGeneratedAsUnannotated explicitly as a way to relax our checks around
thrift generated code which does have @Nullable annotations). Thus, the one
remaining answer seems to be that, in this configuration, generated code should
be treated optimistically for both onOverrideMayBeNullExpr and dataflow.

[coveralls]

Pull Request Test Coverage Report for Build #887

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 92.552%

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Files with Coverage Reduction	New Missed Lines	%
../nullaway/src/main/java/com/uber/nullaway/dataflow/AccessPath.java	2	95.74%

Totals
Change from base Build #886:	-0.01%
Covered Lines:	4859
Relevant Lines:	5250

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💛 - Coveralls

[msridhar]

Nice find! This must have been tricky to track down