Serialize `#pragma detect_mismatch`.

This is like r262493, but for pragma detect_mismatch instead of pragma comment.
The two pragmas have similar behavior, so use the same approach for both.

llvm-svn: 262506
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 844bdd1..ffb7eed 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -338,6 +338,11 @@
   llvm_unreachable("pragma comment cannot be instantiated");
 }
 
+Decl *TemplateDeclInstantiator::VisitPragmaDetectMismatchDecl(
+    PragmaDetectMismatchDecl *D) {
+  llvm_unreachable("pragma comment cannot be instantiated");
+}
+
 Decl *
 TemplateDeclInstantiator::VisitExternCContextDecl(ExternCContextDecl *D) {
   llvm_unreachable("extern \"C\" context cannot be instantiated");