Add an AST node for __leave statements, hook it up.

Codegen is still missing (and I won't work on that), but __leave is now
as implemented as __try and friends.

llvm-svn: 212425
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 0b77891..dc5619d 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -3285,7 +3285,7 @@
   if (!SEHTryParent)
     return StmtError(Diag(Loc, diag::err_ms___leave_not_in___try));
 
-  return StmtError(Diag(Loc, diag::err_ms___leave_unimplemented));
+  return new (Context) SEHLeaveStmt(Loc);
 }
 
 StmtResult Sema::BuildMSDependentExistsStmt(SourceLocation KeywordLoc,