General usability improvements to generic PDB library.

1. Added some asserts to make sure concrete symbol types don't
   get constructed with RawSymbols that have an incompatible
   SymTag enum value.
2. Added new forwarding macros that auto-define an Id/Sym method
   pair whenever there is a method that returns a SymIndexId.
   Previously we would just provide one method that returned only
   the SymIndexId and it was up to the caller to use the Session
   object to get a pointer to the symbol.  Now we automatically
   get both the method that returns the Id, as well as a method
   that returns the pointer directly with just one macro.
3. Added some methods for dumping straight to stdout that can
   be used from inside the debugger for diagnostics during a
   debug session.
4. Added a clone() method and a cast<T>() method to PDBSymbol
   that can shorten some usage patterns.

llvm-svn: 299831