x/tools/gopls: type error analyzers should have the suggested fix as a message

[stamblerre]

Currently, the title of a type error analysis diagnostic has to match the type error for a type error analyzer to be recognized. However, this has the side effect of making the suggested fix title for a type error analyzer be the same as the message of the type error. This makes the suggested fixes confusing, as they don't say what they are actually doing.