[AST] Store the results in OverloadExpr in a trailing array

Use the newly available space in the bit-fields of Stmt to pack
OverloadExpr, UnresolvedLookupExpr and UnresolvedMemberExpr.

Additionally store the results in the overload set in a trailing array.
This saves 1 pointer + 8 bytes per UnresolvedLookupExpr and
UnresolvedMemberExpr.

Differential Revision: https://reviews.llvm.org/D56368

Reviewed By: rjmccall

llvm-svn: 350732