[javasrc2cpg] Populate generic signatures roughly following the class file signature format

[johannescoetzee]

The generic signatures added by this PR follow the format described in https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.7.9.1 and largely correspond to the generic signatures in class files with some notable differences:

Class type signatures

• In most cases, only the simple name for the class will be used (so LString; will be used instead of Ljava/lang/String)
• Where a qualified name is used in source, that name is used verbatim in the signature, for example Ljava.util.List (note the . were not substituted for /.
• For local classes, the name of the class as it appears in the CPG is used in the signature for instances of that class (we don't follow the JVM naming scheme for these), for example Ltestpackage.TestClass.testMethod.LocalClass;

Type parameter bounds

From the language specification:

TypeParameter:
  Identifier ClassBound {InterfaceBound}

ClassBound:
 : [ReferenceTypeSignature]

InterfaceBound:
  : ReferenceTypeSignature

If a type parameter only has interface bounds I1, I2, ..., then the signature should contain <T::LI1;:...> (note the empty class bound), but in general we won't know if a type is a class or interface without resolving the it, so the signature in the CPG will contain <T:LI1;:...> instead.

Unspecified types

Where no type name is specified, the special L__unspecified_type; type is used in generic signatures. This happens in a few places:

• For lambda return types and lambda parameters which do not have explicit type annotations
• For locals with a var type, for example var x = 42
• For synthetic locals created for foreach loops, for example in for (String item : items()), we create a temporary String[] $iterLocal0 = items() local which will have an unspecified signature (item will still have the signature LString; as expected)
• For synthetic locals created for the LHS of instanceof expressions with pattern matching, for example in foo() instanceof String s, we create an Object o = foo() local (since the type depends on the return type of foo).

[ml86]

Please put the description from the PR to into the GenericSignatureTests file so that we are aware of the deviations.

[ml86]

Isnt a single L__unspecified_type more appropriate?