[clang] Fix single-element array initialization in constexpr

https://reviews.llvm.org/D130791 added an improvement that in case array
element has a trivial constructor, it is evaluated once and the result is
re-used for remaining elements. Make sure the constructor is evaluated
for single-elements arrays too.

Fixes https://github.com/llvm/llvm-project/issues/60803

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D145486