Implement support for dependent Microsoft __if_exists/__if_not_exists
statements. As noted in the documentation for the AST node, the
semantics of __if_exists/__if_not_exists are somewhat different from
the way Visual C++ implements them, because our parsed-template
representation can't accommodate VC++ semantics without serious
contortions. Hopefully this implementation is "good enough".

llvm-svn: 142901
diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index c56931b..6f020cc 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -73,6 +73,7 @@
   case Stmt::CXXCatchStmtClass:
   case Stmt::SEHExceptStmtClass:
   case Stmt::SEHFinallyStmtClass:
+  case Stmt::MSDependentExistsStmtClass:
     llvm_unreachable("invalid statement class to emit generically");
   case Stmt::NullStmtClass:
   case Stmt::CompoundStmtClass: