[AST] Store the OwnedTagDecl as a trailing object in ElaboratedType.

The TagDecl *OwnedTagDecl in ElaboratedType is quite commonly
null (at least when parsing all of Boost, it is non-null for only about 600
of the 66k ElaboratedType). Therefore we can save a pointer in the
common case by storing it as a trailing object, and storing a bit in the
bit-fields of Type indicating when the pointer is null.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D50715

llvm-svn: 339862