Diagnose function template definitions inside functions

The parser can only be tricked into parsing a function template
definition by inserting a typename keyword before the function template
declaration. This used to make us crash, and now it's fixed.

While here, remove an unneeded boolean parameter from ParseDeclGroup.
This boolean always corresponded to non-typedef declarators at file
scope. ParseDeclGroup already has precise diagnostics for the function
definition typedef case, so we can let that through.

Fixes PR21839.