Fix crash when using a default argument from a base template

[dpaoliello]

Even if ParamVarDecl::hasDefaultArg returns true, the default argument can be one of three kinds: normal, unparsed and uninstantiated. Each of these kinds needs to be handled differently:

• Normal: get the default arg via getDefaultArg
• Unparsed: treat as not having a default arg.
• Uninstantiated: get the default arg via getUninstantiatedDefaultArg

This change handles all three kinds and centralizes the checks for and calls to get default args to a single location per method.