Currently, there are many instances where SourceLocation objects are
converted to raw representation to be stored in structs that are
used as fields of tagged unions.
This is done to make the corresponding structs trivial.
Triviality allows avoiding undefined behavior when implicitly changing
the active member of the union.
However, in most cases, we can explicitly construct an active member
using placement new. This patch adds the required active member
selections and replaces SourceLocation-s represented as
unsigned int with proper SourceLocation-s.
One notable exception is DeclarationNameLoc: the objects of this class
are often not properly initialized (so the code currently relies on
its default constructor which uses memset). This class will be fixed
in a separate patch.