[ASTImporter] Set MustBuildLookupTable on PrimaryContext

Summary: SetMustBuildLookupTable() must always be called on a primary context.

Reviewers: labath, shafik, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411

Differential Revision: https://reviews.llvm.org/D54863

llvm-svn: 347575