[clang] require template arg list after template kw (#80801)

Require a template argument list after an identifier prefixed by the
template keyword. Introduced by [CWG
96](https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#96),
but the current wording of
[[temp.names]p5](https://eel.is/c++draft/temp.names#6) was introduced in
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html

),
and became [temp.names]p6 somewhere else.

Fixes #53095

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Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>