Modified ASTContext::getTagDeclType() to accept a NULL pointer for the passed

in TagDecl*. This allows the deserializer to use ASTContext to create the
TagTypes. Deserialize TagTypes then rely on pointer-backpatching to resolve
the decls.

This may not be the interface that we want, but as the implementation of
TagTypes will potentially change significantly in the future, I'm leaving this
for now. An appropriate FIXME is in place.

llvm-svn: 44089