[flang] Add more support for alternate returns

Add `hasAlternateReturns` to `evaluate::ProcedureRef`.

Add `HasAlternateReturns` to test subprogram symbols.

Fix `label01.F90` test: It was checking that "error: " didn't appear in
the output. But that was erroneously matching a warning that ends
"would be in error:". So change it to check for ": error: " instead.

Differential Revision: https://reviews.llvm.org/D83007