Add CodeGen support for functions that always return arguments via a new parameter attribute 'returned', which is taken advantage of in target-independent tail call opportunity detection and in ARM call lowering (when placed on an integral first parameter).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179925 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLLexer.cpp

@@ -582,6 +582,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(optsize);
   KEYWORD(readnone);
   KEYWORD(readonly);
+  KEYWORD(returned);
   KEYWORD(returns_twice);
   KEYWORD(signext);
   KEYWORD(sret);