Diagnose missing 'template' keywords in more cases.

We track when we see a name-shaped expression followed by a '<' token
and parse the '<' as a comparison. Then:

 * if we see a token sequence that cannot possibly be an expression but
   can be a template argument (in particular, a type-id) that follows
   either a ',' or the '<', diagnose that the '<' was supposed to start
   a template argument list, and
 * if we see '>()', diagnose that the '<' was supposed to start a
   template argument list.

This only changes the diagnostic for error cases, and in practice
appears to catch the most common cases where a missing 'template'
keyword leads to parse errors within a template.

Differential Revision: https://reviews.llvm.org/D48571

llvm-svn: 335687