Implement p0292r2 (constexpr if), a likely C++1z feature.

llvm-svn: 273602
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index a70b38f..9a4e4a2 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -4981,7 +4981,8 @@
   if (!ToElseStmt && S->getElse())
     return nullptr;
   return new (Importer.getToContext()) IfStmt(Importer.getToContext(),
-                                              ToIfLoc, ToConditionVariable,
+                                              ToIfLoc, S->isConstexpr(),
+                                              ToConditionVariable,
                                               ToCondition, ToThenStmt,
                                               ToElseLoc, ToElseStmt);
 }