Sema: quote mark name (NFC)

Visual Studio improperly tries to parse the unknown pragma.  Quote the parameter
to allow building.

  lib\Sema\TypeCheckDeclObjC.cpp(970): error C2018: unknown character '0x40'
diff --git a/lib/Sema/TypeCheckDeclObjC.cpp b/lib/Sema/TypeCheckDeclObjC.cpp
index 967e4c8..41c97e1 100644
--- a/lib/Sema/TypeCheckDeclObjC.cpp
+++ b/lib/Sema/TypeCheckDeclObjC.cpp
@@ -965,7 +965,7 @@
   }
 }
 
-#pragma mark @objc declaration handling
+#pragma mark "@objc declaration handling"
 
 /// Whether this declaration is a member of a class extension marked @objc.
 static bool isMemberOfObjCClassExtension(const ValueDecl *VD) {