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

llvm-svn: 273602
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index abb885e..bff056e 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -184,6 +184,7 @@
 
 void ASTStmtReader::VisitIfStmt(IfStmt *S) {
   VisitStmt(S);
+  S->setConstexpr(Record[Idx++]);
   S->setConditionVariable(Reader.getContext(),
                           ReadDeclAs<VarDecl>(Record, Idx));
   S->setCond(Reader.ReadSubExpr());