Document that GetTypeForDeclarator() cannot return a null type.

Also add a few asserts for this.  The existing code assumes this in a bunch
of places already (see e.g. the assert at the top of ParseTypedefDecl(), and
there are many unchecked calls on the result of GetTypeForDeclarator()), and
from looking through the code this should always be true from what I can tell.
This allows removing ASTContext::getNullTypeSourceInfo() too as that's now
unused.

No behavior change intended.

llvm-svn: 225125