JSpecify: preserve explicit nullability annotations on type variables when performing substitutions

[msridhar]

Fixes #1091

Consider the following code:

abstract class Test {
  abstract <V> @Nullable V foo(Function<@Nullable V, @Nullable V> f);
  void testPositive(Function<String, String> f) {
     this.<String>foo(f); // error
  }
} 

The call to foo should not type check. Since the type of its parameter f is Function<@Nullable V, @Nullable V>, with explicit @Nullable annotations on the type variables, any Function passed to foo must have @Nullable type arguments. In typechecking this code, NullAway previously substituted the type arguments for the type variables in foo just using built-in javac routines. But, this would yield a formal parameter type Function<String, String>, as the javac routine would not retain the explicit type arguments in the right places. So we would miss reporting an error. This PR fixes the substitutions and re-introduces the annotations on type variables, so we get the type Function<@Nullable String, @Nullable String> for the formal parameter at the call, and report an error correctly.

The main logic changes are in TypeSubstitutionUtils. We add a new RestoreNullnessAnnotationsVisitor and use it to restore nullability annotations from type variables after performing a substitution.

We also extract the TypeMetadataBuilder logic to a top-level source file, and add new methods as needed for this PR. Some of this could have been split into a separate PR but it's a bit of a pain to extract it now.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 40 lines in your changes missing coverage. Please review.

Project coverage is 88.12%. Comparing base (a1df1c4) to head (9674e64).

Files with missing lines	Patch %	Lines
...om/uber/nullaway/generics/TypeMetadataBuilder.java	70.78%	26 Missing ⚠️
.../uber/nullaway/generics/TypeSubstitutionUtils.java	82.05%	5 Missing and 9 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1143      +/-   ##
============================================
- Coverage     88.25%   88.12%   -0.13%     
- Complexity     2264     2265       +1     
============================================
  Files            85       86       +1     
  Lines          7320     7427     +107     
  Branches       1463     1483      +20     
============================================
+ Hits           6460     6545      +85     
- Misses          432      445      +13     
- Partials        428      437       +9     

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[akshayutture]

What kind of substitution are we referring to here exactly (is it for generic types?)? And why are nullability annotations from type variables lost after performing a substitution - Could you clarify more in the diff description?

[akshayutture]

Is the passing around of the config variable everywhere required for this bug fix? I don't see it being used anywhere.

Or is it an unrelated change which has been combined with this PR? If so, can we clarify this in the PR description?