Improve error recovery for missing 'template' keyword in contexts where the

template is valid with or without it (with different meanings).

If we see "dependent.x<...", and what follows the '<' is a valid expression,
we must parse the '<' as a comparison rather than a template angle bracket.
When we later come to instantiate, if we find that the LHS of the '<' actually
names an overload set containing function templates, produce a diagnostic
suggesting that the 'template' keyword was missed rather than producing a
mysterious diagnostic saying that the function must be called (and pointing
at what looks to already be a function call!).

llvm-svn: 304852