Added Vagrant file for Linux CI build bot.
Implemented support for pack expansion types in the AST converter.
Implemented support for decayed types in the AST converter.
Fixed InjectedClassNameType layout in the AST.

* Template specialization member is now a pointer so null value works properly
Improved type loc handling when walking types.

We now keep desugaring them until we get to the proper type loc class.
Do not use getFileEntryForID when setting an OriginalPtr for units.

It might not return proper values for <invalid> TUs.
Improve error handling when looking for symbols with Options.CheckSymbols.