[flang] Fix build error using clang

Change `AllFortranNames()` to return a `std::vector` rather than a
`std::initialization_list`. The latter doesn't own its underlying
storage and so can't be returned as a value. clang detects this and
issues a warning.

Two tests in `resolve63.f90` behave differently with clang and require
further investigation.

Original-commit: flang-compiler/f18@1ed3a3cfee1d372b23afc4529d5867ecd46db681
Reviewed-on: https://github.com/flang-compiler/f18/pull/810