Escape more @ signs in Doxygen comments.

Doxygen treats "@command" the same as "\command" in a doc comment, so
whenever we talk about Objective-C things like "@interface" we have to
make sure to escape them.

Let's try to keep Clang -Wdocumentation-clean!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178603 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index beae217..16887da 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -10807,8 +10807,8 @@
 }
 
 /// ActOnLastBitfield - This routine handles synthesized bitfields rules for 
-/// class and class extensions. For every class @interface and class 
-/// extension @interface, if the last ivar is a bitfield of any type, 
+/// class and class extensions. For every class \@interface and class 
+/// extension \@interface, if the last ivar is a bitfield of any type, 
 /// then add an implicit `char :0` ivar to the end of that interface.
 void Sema::ActOnLastBitfield(SourceLocation DeclLoc,
                              SmallVectorImpl<Decl *> &AllIvarDecls) {