giving them rough classifications (normal types, never-canonical
types, always-dependent types, abstract type representations) and
making it far easier to make sure that we've hit all of the cases when
decoding types. 

Switched some switch() statements on the type class over to using this
mechanism, and filtering out those things we don't care about. For
example, CodeGen should never see always-dependent or non-canonical
types, while debug info generation should never see always-dependent
types. More switch() statements on the type class need to be moved 
over to using this approach, so that we'll get warnings when we add a
new type then fail to account for it somewhere in the compiler.

As part of this, some types have been renamed:

  TypeOfExpr -> TypeOfExprType
  FunctionTypeProto -> FunctionProtoType
  FunctionTypeNoProto -> FunctionNoProtoType

There shouldn't be any functionality change...

llvm-svn: 65591