Call setMustBuildLookupTable on TagDecls in ExternalASTMerger

Summary:
setMustBuildLookupTable should be called on imported TagDecls otherwise we may fail
to import their member decls (if they have any).

Not calling the setMustBuildLookupTable method results in a failure in the attached test
case when lookup for the 'x' member fails on struct S, which hasn't had its decls imported
elsewhere. (By contrast the member-in-struct testcase hasn't run into this issue
because the import of its decls is triggered when the struct instance is defined, and the
member access follows this).

Reviewers: spyffe, rsmith

Reviewed By: spyffe, rsmith

Differential Revision: https://reviews.llvm.org/D34253

llvm-svn: 305619