[Clang] Fix PR28101

Fixes https://github.com/llvm/llvm-project/issues/28475 (PR28101)
by setting identifier for invalid member variables with template parameters,
so that the invalid declarators would not crash clang.

See also: https://github.com/llvm/llvm-project/commit/942c03910a

Differential Revision: https://reviews.llvm.org/D115248