When we see a '<' operator, check whether it's a probable typo for a template-id.

When we see a '<' operator, check whether it's a probable typo for a template-id.

The heuristic that we use here is:
 * the left-hand side must be a simple identifier or a class member access
 * the right-hand side must be '<' followed by either a '>' or by a type-id that
   cannot be an expression (in particular, not followed by '(' or '{')
 * there is a '>' token matching the '<' token

The second condition guarantees the expression would otherwise be ill-formed.

If we're confident that the user intended the name before the '<' to be
interpreted as a template, diagnose the fact that we didn't interpret it
that way, rather than diagnosing that the template arguments are not valid
expressions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302615 91177308-0d34-0410-b5e6-96231b3b80d8