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/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 0579bca..4c9ccd7 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -8505,6 +8505,8 @@
       return false;
   } else if (isa<PragmaCommentDecl>(D))
     return true;
+  else if (isa<PragmaDetectMismatchDecl>(D))
+    return true;
   else if (isa<OMPThreadPrivateDecl>(D))
     return true;
   else