Refactor: Extract two separate parts from JetFunctionPlatformDescriptorImpl

Rename: JetFunctionPlatformDescriptor -> JetMethodDescriptor
Rename usages
++
Refactor: JetChangeSignatureData object is created inside JetChangeSignatureUtil#runChangeSignature call
-- PREP
Minor: Remove commented out code
Rename: OverridingUtil#filterOverrides -> filterOutOverridden to be more precise
Add filterOutOverriding to OverridingUtil
RENAME: some changes