fix Issue 10558 - Assertion failure on struct.c:741

A template instance on temlpate argument may not have yet valid `Dsymbol::ident`, so should use `Dsymbol::getIdent()` for hash calculation.
Merge pull request #2314 from 9rnsr/fix10558

[REG2.064a] Issue 10558 - Assertion failure on struct.c:741