Intra-TU merging of `Info` needs work

[sdkrystian]

The merging of Info that occurs during AST traversal is quite lacking. For example, we do not update function parameter names or default arguments when we encounter redeclarations; the parameters are only extracted from the first declaration. We also should do this for types when e.g. a first declaration uses a type T and a redeclaration uses a typedef-name U which denotes T; in such cases "sweeten" the type by merging cv-qualifiers and applying them to the most-sugared type