Improve recovery for template-ids whose template-name doesn't actually

name a template, when they occur in a base-specifier. This is one of
the (few) places where we know for sure that an identifier followed by
a '<' must be a template name, so we can diagnose and recover well:

test/SemaTemplate/dependent-base-classes.cpp:9:16: error: missing
'template'
      keyword prior to dependent template name 'T::apply'
struct X1 : T::apply<U> { }; // expected-error{{missing 'template' ...
               ^
               template 
test/SemaTemplate/dependent-base-classes.cpp:12:13: error: unknown
template name
      'vector'
struct X2 : vector<T> { }; // expected-error{{unknown template name
'vector'}}
            ^
2 diagnostics generated.

llvm-svn: 93257