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/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 91bc809..a572687 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -642,6 +642,7 @@
     case StaticAssert:
     case ObjCPropertyImpl:
     case PragmaComment:
+    case PragmaDetectMismatch:
     case Block:
     case Captured:
     case TranslationUnit: