[clang][AST] Check context of record in structural equivalence.

The AST structural equivalence check did not differentiate between
a struct and a struct with same name in different namespace. When
type of a member is checked it is possible to encounter such a case
and wrongly decide that the types are similar. This problem is fixed
by check for the namespaces of a record declaration.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D113118