Fix uninitialized variable in GoParser

Summary:
`m_last_tok` isn't initialized anywhere before it's used the first time (most likely in the `GoParser::Rule::error` method), which causes most of the GoParser tests to fail with sanitizers enabled with errors like this:

```
GoParser.cpp:52:21: runtime error: load of value <random value>, which is not a valid value for type 'GoLexer::TokenType'
UndefinedBehaviorSanitizer: undefined-behavior GoParser.cpp:52:21
```

Reviewers: ribrdb, davide, labath

Reviewed By: labath

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D42339

llvm-svn: 323119