[flang] Nonconformant assigned gotos

Modify code generation for assigned gotos to generate a runtime error
for most cases that violate F90 Clause 8.2.4, rather than treating a
nonconformant GOTO as a nop. For example, generate a runtime error for
a GOTO that attempts to branch to a label for a FORMAT statement.
Relax the requirement that an assigned GOTO with a label list must
branch to a label in the list, and instead allow a branch to any valid
assigned GOTO target in scope.