Interfaces that were sealed: KSAnnotated, KSDeclarationContainer,
KSReferenceElement, KSModifierListOwner, KSPropertyAccessor,
KSDeclaration and PlatformInfo

Abstract implementation classes that inherited from now sealed types
have those supertypes removed and replace overridden with open members.
Additional casts had to be introduced in a few places.

A few when statements had to be made exhaustive (by specifying all
cases) because they now operate on subjects with a sealed type.

KSNodeDescriptorImpl, KSNodeJavaImpl and KSNodeKtImpl were removed
because they were unused but inherited from the now sealed KSNode.

# Conflicts:
#	common-util/src/main/kotlin/com/google/devtools/ksp/common/IncrementalContextBase.kt
#	kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/AbstractKSDeclarationImpl.kt
#	kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSTypeAliasImpl.kt

# Conflicts:
#	kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/AbstractKSDeclarationImpl.kt