[clang] Properly print unnamed members in diagnostics

Use member's type when printing.
This also fixes a bug in warning diagnostic for out of order
initialization with designated initializers so it points to a valid
source location when an anonymous member is being initialized.

Fixes https://github.com/llvm/llvm-project/issues/63759

Reviewed By: aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D158526