[flang] Simplify Semantics::Perform

`Semantics::Perform` is mostly a series of calls followed by a check
for fatal errors. There is more error checking logic than real code.

To make it clearer, change each of the phases it calls to return true
on success so that `Perform` can just call them one after the other.

Original-commit: flang-compiler/f18@a218cac788fdc4018645651964e75221c6f31653
Reviewed-on: https://github.com/flang-compiler/f18/pull/317