Emit SemanticDB types and signatures

[Strum355]

Emit Type and Signature messages for symbol occurrences.
With this information, we can reconstruct accurate hover signatures when converting from SemanticDB to LSIF.

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Footnotes

Wildcard (existential) types proved most difficult to decipher. My work here is a combination of scalameta Javacp.scala file, the semanticdb spec ("Hardlinking comes in handy in situations when an advanced type such as ... ExistentialType") and this one table entry from the semanticdb spec (in which the params are the opposite way around):

As wildcard params dont have a symbol emitting for them, I create a hardlink as opposed to a symlink in the declarations field of the existential type. In a mixed wildcard and non-wildcard scenario, the declarations ordering must be derived from the ExistentialType.tpe.type_arguments field, as in the case of Triplet<?, String, ? extends Integer>, the ordering of declarations would be symlink: "String", hardlinks: "local_wildcard", hardlinks: "local_wildcard"+upper_bound: "java/lang/Integer#". The relative ordering of symlinks to symlinks and hardlinks to hardlinks is preserved, just not symlinks to hardlinks and vice versa, so there should be no problem there.

I had to do some guessing as to the behaviour when theres multiple wildcard type params for a given type (as is the case with e.g. Map<?, ?>), which isn't covered in the semanticdb spec, but this seems fairly sane.

Decoded Protobuf

Link to Java source for below: https://github.com/sourcegraph/lsif-java/pull/126/files#diff-3f725eecc09c1818a18120a2d471018890dd664200b233dcbee8c14c650824aeR10

  symbols {
    symbol: "minimized/ParameterizedTypes#doStuff()."
    signature {
      method_signature {
        type_parameters {
        }
        parameter_lists {
        }
        return_type {
          existential_type {
            tpe {
              type_ref {
                symbol: "java/util/Map#"
                type_arguments {
                  type_ref {
                    symbol: "local_wildcard"
                  }
                }
                type_arguments {
                  type_ref {
                    symbol: "local_wildcard"
                  }
                }
              }
            }
            declarations {
              hardlinks {
                symbol: "local_wildcard"
                signature {
                  type_signature {
                    type_parameters {
                    }
                    upper_bound {
                      type_ref {
                        symbol: "java/lang/String#"
                      }
                    }
                  }
                }
              }
              hardlinks {
                symbol: "local_wildcard"
                signature {
                  type_signature {
                    type_parameters {
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }

[olafurpg]

This is exciting progress! The implementation looks good to me. Just missing the pretty-printer before we can merge this. It's best to wait until the lsif-semanticdb PR is merged before we integrate the pretty-printer into LSIF hover messages