[flang] Continue semantics checks after expression error

Change statement semantics to continue with checks after an error
in expression analysis. This allows the compiler to report more
compilation errors. It requires that statement semantics not assume
that every parser::Expr has a valid evaluate::Expr.

For example, the test cases in coarrays02.f90 can now be moved to
coarrays01.f90. Previously the errors like "Must be a scalar value"
from ExprChecker prevented other errors from being detected by
CoarrayChecker.

Change to a stable sort of messages so that they come out in a
deterministic order. Otherwise when there are two errors at the
same location (e.g. line 71 of coarrays01.f90) the test can fail
randomly.

Original-commit: flang-compiler/f18@f420d219096c0f40c17a07d926122840669debb4
Reviewed-on: https://github.com/flang-compiler/f18/pull/422
Tree-same-pre-rewrite: false