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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305619 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/ExternalASTMerger.cpp

@@ -41,6 +41,7 @@ public:
   Decl *Imported(Decl *From, Decl *To) override {
     if (auto ToTag = dyn_cast<TagDecl>(To)) {
       ToTag->setHasExternalLexicalStorage();
+      ToTag->setMustBuildLookupTable();
     } else if (auto ToNamespace = dyn_cast<NamespaceDecl>(To)) {
       ToNamespace->setHasExternalVisibleStorage();
     }

test/Import/indirect-struct-member-access/Inputs/S.c

+struct S {
+  int a;
+};

test/Import/indirect-struct-member-access/test.c

+// RUN: clang-import-test -import %S/Inputs/S.c -expression %s
+void expr(struct S *MyS) {
+  MyS->a = 3;
+}