Adding support for nullable type upper bounds considering Library models

[akulk022]

The below line previously reported an error considering the upperbound of the generic type parameter couldn't be Nullable, adding it into library models to allow it to be Nullable passes the test case.

Function<String,@Nullable String> removeA = a -> a.replace("a","");

We now have library model methods for treating a type as @NullMarked and also for making the upper bound of type variables @Nullable. We use these new methods to model java.util.function.Function as @NullMarked, in JSpecify mode only.

We fixed a related issue with unwanted errors when passing a lambda / method reference to @NullUnmarked code; this was necessary to get NullAway itself to still build with NullAway checking enabled.

(Due to issues with the branch this PR was recreated from the original #893)

[msridhar]

Another issue, see below

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (3fc9b8a) 86.98% compared to head (d55b045) 87.05%.
Report is 3 commits behind head on master.

Files	Patch %	Lines
...c/main/java/com/uber/nullaway/NullabilityUtil.java	66.66%	1 Missing and 2 partials ⚠️
...away/src/main/java/com/uber/nullaway/NullAway.java	93.75%	0 Missing and 2 partials ⚠️
...ava/com/uber/nullaway/generics/GenericsChecks.java	88.88%	0 Missing and 2 partials ⚠️
...src/main/java/com/uber/nullaway/LibraryModels.java	66.66%	1 Missing ⚠️
...llaway/dataflow/AccessPathNullnessPropagation.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #903      +/-   ##
============================================
+ Coverage     86.98%   87.05%   +0.07%     
- Complexity     1959     1988      +29     
============================================
  Files            77       77              
  Lines          6330     6414      +84     
  Branches       1223     1245      +22     
============================================
+ Hits           5506     5584      +78     
- Misses          420      422       +2     
- Partials        404      408       +4     

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[msridhar]

Going ahead and merging this one. Thanks again @akulk022!