Describe the PR

Add a VoidType node to replace ResultType. This means we don't need the ResultType wrapper when the method is not void, and the result type node is never null:

void foo();

└─ MethodDeclaration
   └─ ResultType[@Void=true()]
   └─ MethodDeclarator
      └─ FormalParameters

└─ MethodDeclaration
   └─ ModifierList
   └─ VoidType
   └─ FormalParameters

int foo();

└─ MethodDeclaration
   └─ ResultType[@Void=false()]
      └─ Type
         └─ PrimitiveType
   └─ MethodDeclarator
      └─ FormalParameters

└─ MethodDeclaration
   └─ ModifierList
   └─ PrimitiveType
   └─ FormalParameters

This also makes the qualifier of a ClassLiteral non-null, which removes the inconsistency between void.class and T.class:

void.class

ClassLiteral[@Void=true()]

└─ ClassLiteral
   └─ VoidType

int.class

└─ ClassLiteral
   └─ PrimitiveType

└─ ClassLiteral
   └─ PrimitiveType

(this change description is relative to java-grammar as ASTClassLiteral does not exist on master, so we don't have to put it into the clean release notes).

Related issues

• Fixes #

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by travis)
• Added (in-code) documentation (if needed)